Donald Trump's campaign chairman has resigned from his post less than three months before Election Day, the Republican presidential nominee announced on Friday.

"This morning Paul Manafort offered, and I accepted, his resignation from the campaign," Trump said in a statement. "I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process."

Manafort joined the campaign in mid-March as a chief strategist and delegate czar. He was promoted to campaign chairman in May, shortly after Trump's remaining primary opponents exited the race, leaving him to become the presumptive GOP presidential nominee.

The 67-year-old Washington insider was well-respected by Trump's grown children — Ivanka, Eric and Donald Jr. — for helping to convert the billionaire's campaign into a more traditional operation.

He met often with GOP lawmakers on Capitol Hill, led outreach efforts to the party's establishment wing and worked closely with Republican National Committee Chairman Reince Priebus to improve the campaign's ground game and the candidate's message. When tension emerged between him and ex-campaign manager Corey Lewandowski, it was Manafort Trump's children encouraged their father to keep around.

Lewandowski suggested that Manafort's resignation was due to differences between his vision for the campaign and Trump's, but sources close to the campaign say Manafort was satisfied with where he had taken it and will continue to work with the team in an informal capacity.

"At the end of the day, you've got people who had a vision for the campaign, which did not align with what Donald Trump wanted," Lewandowski told CNN's Kate Bolduan on Friday. "I mean, it's been widely reported that there has not ben a robust ground effort in states like Florida [and] you cannot blame the candidate for those things."

Trump, however, praised Manafort — whose resume includes advisory roles on the campaigns of Ronald Reagan, George H.W. Bush and John McCain — on Friday, and the veteran Washington insider has reportedly departed the campaign on good terms.

Still, Manafort's resignation comes just days after multiple reports raised new questions about his ties to pro-Kremlin forces in Ukraine and whether he received off-the-books cash payments from Ukraine's ousted President Viktor Yanukovych, an ally of Russian President Vladimir Putin.

He later described such allegations as "unfounded, silly and nonsensical," and was defended by Trump's running mate, Indiana Gov. Mike Pence, who criticized the media for focusing more on Manafort than on Democratic presidential hopeful Hillary Clinton.

"The truth is Mr. Manafort is involved in our campaign, but he's not running for president," Pence told Fox 28 Columbus on Tuesday.

Manafort leaves the campaign less than 48 hours after Trump promoted Republican pollster Kellyanne Conway to manage his campaign and hired Steve Bannon, the executive chairman of Breitbart News, to serve as CEO.

"Paul is a true professional and I wish him the greatest success," Trump said.