Updated at 12:15 p.m. ET

Republican presidential nominee Donald Trump is shaking up his campaign staff, after a series of missteps that led to slumping poll numbers.

Trump has tapped Stephen Bannon of Breitbart News to serve as chief executive of the campaign. Pollster Kellyanne Conway was promoted to campaign manager. Paul Manafort will stay on as Trump's campaign chairman. The Wall Street Journal first reported the news.

"I have known Steve and Kellyanne both for many years," Trump said in a statement. "They are extremely capable, highly qualified people who love to win and know how to win."

Bannon has been in charge of Breitbart News for a period in which it has been a strong platform for the kind of populism and fierce opposition to illegal immigration that gave rise to Trump's candidacy in the Republican primaries. A campaign release highlighted that Bloomberg Politics has called Bannon the "most dangerous political operative in America."

Since the Republican and Democratic conventions last month, Trump has been losing ground in both national and swing-state polls. NPR's latest analysis of the electoral map suggests if the election were held today, Democratic rival Hillary Clinton would have a clear path to the White House, even if she were to lose the most closely contested states.

Republicans have complained about Trump's repeated stumbles, including his fight with the Gold Star family of a Muslim Army captain who was killed in Iraq and his claim last week that President Obama is the "founder" of ISIS.

Trump appeared somewhat more disciplined in recent days, largely sticking to his script during a counter-terrorism speech in Ohio on Monday and a law-and-order address outside Milwaukee on Tuesday night. At the same time, Trump has shrugged off the value of any sort of pivot, telling Wisconsin television station WKBT on Tuesday, "I am who I am. It's me. I don't want to change. Everyone talks about, 'Oh, well you're gonna pivot, you're gonna' — I don't want to pivot, I mean, you have to be you. If you start pivoting, you're not being honest with people."

A source close to the campaign told NPR that Trump was likely seeking support from "someone who doesn't want to change him, but just wants to make him a better bulldozer operator."

Manafort also came under scrutiny this week after the New York Times reported that his name had appeared in a ledger of under-the-table payments made by former Ukraine President Viktor Yanukovych's party. Manafort, however, dismissed the idea that he'd accepted cash payments for his work in Ukraine as "unfounded, silly and nonsensical."

Manafort has been effectively managing the campaign since June, when Trump's previous campaign manager Corey Lewandowski was dismissed amid a pivot toward the general election. At the time, Trump described the change to Fox News: "I think it's time now for a different kind of a campaign. We ran a small, beautiful, well-unified campaign. It worked very well in the primaries."

Those close to the campaign had described, at that time, two opposing camps behind the scenes aligned to Manafort and Lewandowski. Possible tensions between the two received fresh attention when Lewandowski tweeted out that recent New York Times story about Manafort's ties to the ousted pro-Russian government in Ukraine.

Manafort spoke of a pivot in tone and style in his early days on the campaign, but all along Trump has insisted there will be no change. His more scripted events on the trail in recent days followed weeks of controversy the GOP nominee has inflicted on his campaign with off-the-cuff statements that have ostensibly dragged down his poll numbers since the end of the political conventions.

On Wednesday morning, Lewandowski weighed in on the changes, praising Conway's long experience in Republican campaigns and long-standing ties to Trump. He also described similarities between Bannon's style and his own.

"He's a person who, I think, a little bit like myself, is a bit of a street fighter," Lewandowski said on CNN, where he has been a paid contributor since being dismissed from the Trump campaign in June. He went on to describe Bannon as "a person who is willing to go right at his opponents and make sure that they know that, in politics, all's fair in love and war."

The campaign shakeup comes as the campaign prepares to launch its first television ads of the general election. The Clinton campaign and its allies have already spent more than $100 million on TV ads.

"I am committed to doing whatever it takes to win this election, and ultimately become President," Trump said in his statement. "Our country cannot afford four more years of the failed Obama-Clinton policies which have endangered our financial and physical security."

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