As Donald Trump has focused the messaging of his presidential campaign in recent weeks, he's centered on one key attack on Democrat Hillary Clinton: The suggestion that the Clinton Foundation was a pay-to-play front that enabled Hillary and Bill Clinton to trade government access and favors for money.

"It's impossible to figure out where the Clinton Foundation ends and the State Department begins," Trump said Tuesday night in Texas. "It is now abundantly clear that the Clintons set up a business to profit from public office. They sold access and specific actions by and to them for money."

Bill and Hillary Clinton have defended the foundation's work under the intensifying attacks. "We're trying to do good things," Bill Clinton said Wednesday. "If there's something wrong with creating jobs and saving lives, I don't know what it is. The people who gave the money knew exactly what they were doing. I have nothing to say about it except that I'm really proud."

The Clintons don't directly profit from the foundation, despite Trump's accusation. Neither Bill nor Hillary Clinton draws a salary from it, according to tax documents.

The Clinton Foundation very much mirrors former President Bill Clinton's public image. It is ambitious. It is sprawling and, at times, unfocused. Its practices raise many ethical red flags that political observers say could have easily been avoided. And yet it has also delivered concrete results in public policy fields as wide-ranging as lowering the price of HIV/AIDS medication in sub-Saharan Africa to combating childhood obesity.

What, exactly, does the Clinton Foundation do?

A lot.

The Clinton Foundation is where Bill Clinton focused his efforts after leaving office in 2001. Initially formed to raise funds for and build Clinton's presidential library, the foundation morphed into one of the world's most ambitious nonprofit organizations over the past 16 years.

It has shrunk, grown and been reorganized several times. In its present form, the foundation is a constellation of nine different initiatives and two "affiliated entities," each focused on achieving a different public policy goal.

That includes lowering the cost of HIV/AIDS medication; addressing climate change; improving education in Haiti; and an annual conference that gathers heads of state, world leaders and celebrities to New York City, where they all make philanthropic promises.

The Clintons frequently point to the foundation's Health Access Initiatives, saying they have lowered the cost of HIV/AIDS treatment around the world, and helped deliver cheaper drugs to more than 9 million people. That's a claim PolitiFact rated as true.

The foundation is organized around the idea of convening and connecting. "We believe that the best way to unlock human potential is through the power of creative collaboration," its website states. "That's why we build partnerships between businesses, NGOs, governments and individuals everywhere to work faster, leaner and better."

As The New York Times explained it:

"Instead of handing out grants, the foundation recruits donors and advises them on how best to deploy their money or resources, from helping Procter & Gamble donate advanced water-purification packets to developing countries to working with credit card companies to expand the volume of low-cost loans offered to poor inner city residents."

All those programs require a whole lot of money. The foundation has raised more than $2 billion since 2001, according to The Washington Post.

The foundation also didn't used to be so polarizing. Plenty of Republicans have spoken at the Clinton Global Initiative, including Mitt Romney just 42 days before the 2012 election.

"Since serving as president, President Clinton has devoted himself to lifting people around the world," Romney said then. "And one of the best things to happen to any cause, to any people, is to have Bill Clinton as its advocate."

Even Donald J. Trump gave between $100,001 and $250,000 to the foundation.

So what's the problem?

The problem is all that money — and whether donors were motivated by anything other than pure goodwill and philanthropy. Namely: whether they gave because of the promise of access to and favors from Hillary Clinton, who was just beginning her career in public office at the time that Bill Clinton had ended his.

Every former president has set up some sort of foundation and aggressively raised money to fund it. Only one former president — Bill Clinton — has been married to someone eyeing her own run for the White House.

Emails released this month by conservative advocacy group Judicial Watch highlight several instances during Hillary Clinton's tenure as secretary of state where Clinton Foundation staffers contacted the State Department soliciting favors for people who had donated to the organization.

Here's how The New York Times characterized one email conversation:

"In April 2009, Douglas J. Band, who led the foundation's Clinton Global Initiative, emailed Ms. Abedin and Cheryl D. Mills, another top adviser to Mrs. Clinton, for help with a donor.

"Mr. Band wrote that he needed to connect Gilbert Chagoury, a Lebanese-Nigerian billionaire who was one of the foundation's top donors, with someone at the State Department to talk about his interests in Lebanon.

" 'It's Jeff Feltman,' Ms. Abedin answered, referring to Jeffrey Feltman, who was the American ambassador to Lebanon at the time. 'I'm sure he knows him. I'll talk to Jeff.'

"Mr. Band asked her to call Mr. Chagoury immediately if possible. 'This is very important,' he wrote."

In another exchange that year, Band emailed Huma Abedin — a close Clinton aide who was working at the State Department at the time — to request that Clinton meet with Crown Prince Salman of Bahrain, whom he referred to as a "good friend of ours."

She responded:

From: Huma Abedin

To: Doug Band

Sent: Tue Jun 23 4:12:46 2009

Subject: Re:

He asked to see hrc thurs and fri thru normal channels. I asked and she said she doesn't want to commit to anything for thurs or fri until she knows how she will feel. Also she says that she may want to go to ny and doesn't want to be committed to stuff in ny...

Abedin then followed up, offering the crown prince a sitdown with Clinton. "If u see him, let him know," she wrote Band. "We have reached out through official channels."

At a 2005 Clinton Global Initiative event, Salman committed $32 million to youth scholarship programs. That wasn't a donation to the Clinton Foundation itself — just a promise to spend the money to help "Bahraini students to take leadership roles in the private and public sectors."

Additionally, the Kingdom of Bahrain has donated more than $50,000 to the Clinton Foundation itself, according to the group's records.

The Clinton campaign has repeatedly denied that the Department of State took any action in response to contributions to the Clinton Foundation.

What's this about Secretary Clinton meeting with foundation donors, though?

Republicans' attacks on the Clinton Foundation intensified this week after The Associated Press published an article claiming that "more than half" of the private citizens Clinton met with as secretary of state were foundation donors.

"It's an extraordinary proportion indicating her possible ethics challenges if elected president," the article stated.

Except that's not quite the full story. The AP ignored the thousands of meetings that Clinton held with government officials from the United States and other countries. "Such meetings would presumably have been part of her diplomatic duties," the news organization argued.

Clinton's campaign said that framing created an unfair picture. "This story relies on utterly flawed data," Clinton spokesman Brian Fallon said in a statement. "It cherry-picked a limited subset of Secretary Clinton's schedule to give a distorted portrayal of how often she crossed paths with individuals connected to charitable donations to the Clinton foundation."

Fallon said there were more than 1,700 Clinton meetings that were not factored in.

Still, no matter what the total percentage was, Clinton still sat down 40 people who had donated more than $100,000 to the Clinton Foundation, and 20 who had donated more than $1 million, according to the AP's tallies.

There's no question the optics are bad for Clinton and the Clinton Foundation. But no proof has emerged that any official favors — regulations, government contracts, international deals — were curried in exchange for donations or pledges.

One case that Clinton critics point to involves banking giant UBS, which was locked in a tense battle with the Internal Revenue Service when Clinton took office in 2009. The Wall Street Journal published a detailed look at Clinton's dealings with the bank:

"The Internal Revenue Service was suing UBS AG to get the identities of Americans with secret accounts. If the case proceeded, Switzerland's largest bank would face an impossible choice: Violate Swiss secrecy laws by handing over the names, or refuse and face criminal charges in U.S. federal court.

"Within months, Mrs. Clinton announced a tentative legal settlement — an unusual intervention by the top U.S. diplomat. UBS ultimately turned over information on 4,450 accounts, a fraction of the 52,000 sought by the IRS, an outcome that drew criticism from some lawmakers who wanted a more extensive crackdown.

"From that point on, UBS's engagement with the Clinton family's charitable organization increased. Total donations by UBS to the Clinton Foundation grew from less than $60,000 through 2008 to a cumulative total of about $600,000 by the end of 2014, according to the foundation and the bank."

The bank also paid Bill Clinton $1.5 million for a series of private speeches.

But it's not like the UBS standoff wasn't already a top diplomatic concern for Clinton. According to The Journal, it was the very first issue Switzerland's foreign minister brought up with Clinton when the two met in the early months of Clinton's tenure.

Sorting out the overlapping lines of the Clinton Foundation and the State Department are murky, because the types of people and groups who are apt to make major donations to a top-tier international nonprofit are the same types of people and groups who would seek out — and be granted — meetings with top State Department officials.

The Clinton campaign pointed this out in its response to the AP story. The AP included Melinda Gates, for example, in its tally of donors whom Clinton met with while at State.

"Melinda Gates is a world-renowned philanthropist whose foundations works to address global health crises and eradicate disease in the developing world," Fallon said in his statement. "Meeting with someone like Melinda Gates is squarely in the purview of America's top diplomat, whose job involves confronting these game global challenges."

'Unraveling' the foundation

When Clinton joined the Obama administration, the Obama Transition Team put together a memorandum of understanding that aimed at unblurring the lines between Clinton's official duties at the State Department, and the work her family's foundation was carrying out in similar fields.

The foundation agreed to disclose its donors on an annual basis. It also spun off the Clinton Global Initiative — that annual confab of world leaders — and its HIV/AIDS Initiative into separate, but still related, entities. The foundation also agreed to inform the State Department of any foreign contributions.

Still, it was hard to keep everything separate. Take the two Clinton staffers from those email exchanges: Doug Band and Huma Abedin. Toward the end of Clinton's tenure, Abedin sought and was granted "special government employee" status, which meant she could work part time for both the State Department and outside groups. She ended up doing work for the Clinton Foundation, as well as Teneo Holdings, a consulting firm run by Band.

And while Band, a longtime aide to Bill Clinton, never worked for State, he also held multiple roles within the Clintons' orbit. As Eric Lichtblau of The New York Times recently explained it to NPR:

"He is a personal assistant to Bill Clinton. He ran a consulting company that paid Bill Clinton a lot of money until 2012. He is also the head of Clinton's Global Initiative which gets together muckety mucks from all over the world for big charity events, and he's with the Clinton Foundation. So to figure out at any one time which hat someone like Doug Band is wearing when he communicates with aides to Hillary Clinton is a bit of a puzzle."

The Clinton Foundation recently announced changes that would take place if Hillary Clinton is elected president. Bill Clinton would step down from its board; the foundation would stop accepting foreign money; the Clinton Global Initiative would end after next month.

And, NPR learned Tuesday, the foundation will work to "spin off" many of its international efforts.

"The most important thing to the president and to Chelsea is that the work continues but under different umbrellas away from the foundation, obviously, because we will not be able to accept corporate donations or international money as we have to support our programs," foundation President Donna Shalala told NPR.

That announcement has not satisfied Trump and other Republicans, who are now calling — on a near-daily basis — for the Clintons to shut the entire foundation down.

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