(Note: Tonight's debate, moderated by PBSNewsHouranchors Gwen Ifill and Judy Woodruff, will be simulcast on CNN and NPR and streamed live onNPR.org. NPR's Tamara Keith will be part of the debate broadcast, providing analysis during and after the event.)

Bernie Sanders and Hillary Clinton meet Thursday night on a debate stage in Milwaukee. It's their first face-to-face matchup since Tuesday's New Hampshire primary where Sanders beat Clinton by more than 20 points.

Sanders raised more than $5 million in just 18 hours after polls closed (average contribution, according to his campaign: $34), giving his campaign a rocket boost of money to match the energy of his supporters. Clinton is hoping to slow his momentum, and her campaign is working to shore up support among black and Latino voters in Nevada, South Carolina and beyond. So, let's just say the stakes are high in tonight's debate, especially for Clinton. Here are four things to watch for tonight:

1. Will they attack each other?

Clinton and her supporters maintain Sanders has gotten a pass from reporters and Republicans (who have attacked her relentlessly while gleefully playing up Sanders' triumphs). Clinton's campaign is already starting to draw sharper contrasts with the Vermont senator. But she's under pressure to find a way to slow Sanders down, and the first chance she'll get is tonight's debate.

In past debates, Sanders has avoided attacking Clinton directly on her "damned" emails and her high-dollar speaking fees while effectively tying her to Wall Street and corporate interests. This line of attack (or whatever you want to call it) has hurt Clinton.

In exit polls on Tuesday, 91 percent of Democrats who said having a candidate who is "honest and trustworthy" was a top priority voted for Sanders. Many voters NPR interviewed said money in politics and Wall Street ties are what tipped them away from Clinton and toward Sanders. Will he keep it up or add new, more direct lines of attack?

2. Will they modify their messaging to appeal to African-American, Latino and young voters?

The first two states to vote were overwhelmingly white. That dynamic shifts significantly in Nevada, South Carolina and Super Tuesday states — all of which have more African-American and Latino voters. Watch for the candidates to shift the suite of issues they emphasize to speak more directly to those voters. Expect to hear more about the Flint water crisis, criminal justice reform, voting rights and immigration reform.

Meanwhile, Clinton recognized in her concession speech Tuesday night that she needs to improve how she communicates with young voters, who overwhelmingly support Sanders. This will be the first chance for viewers to see how she is retooling her message following the vote in New Hampshire.

3. How far left will the Democratic Party go in this primary?

When Clinton started her campaign, her message was more general election than primary. Now she is running in a tough primary against a democratic socialist who beat her in New Hampshire. Sanders has given her a real race, and she's hitting increasingly populist tones. It may not have been his goal, but Sanders has pulled Clinton to the left. The question now is how far they will go. Debates are always an opportunity for candidates to test each other, and that can sometimes mean saying or agreeing to positions that will give the other party fodder in the general election.

How far left will the Democratic Party go to settle the nomination, and what does that mean for the general election?

4. Will PBS moderate differently than other TV networks have?

PBS is a noncommercial network that doesn't live and die by ratings quite the way CNN and NBC do. NewsHour, anchored by Gwen Ifill and Judy Woodruff, is known for lengthy, thoughtful interviews and stories that dig deep. It seems quite likely that sensibility will change the types of questions that are asked. This could be good news for Democratic voters who haven't yet seen the candidates get beyond soundbite answers in debates on issues like criminal justice reform and immigration.

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