MILWAUKEE (Reuters) - President Joe Biden's reelection bid was mired in fresh turmoil after news reports that top Democratic leaders had privately pushed him to end his campaign, while Donald Trump was set to accept the Republican presidential nomination on Thursday at his party's national convention.

Senate Majority Leader Chuck Schumer, House Minority Leader Hakeem Jeffries and former House Speaker Nancy Pelosi have all expressed deep concerns directly to Biden in recent days that he will not only lose the White House but also cost the party any chance of winning back the U.S. House of Representatives in the Nov. 5 election, according to reports in multiple news outlets.

Biden, 81, has thus far refused to entertain public calls from 20 congressional Democrats to step aside, following a halting performance at his June 27 debate against Trump, 78.

His troubles were compounded on Wednesday when he tested positive for COVID-19 during a campaign visit to Nevada, forcing him to return to his Delaware home to work in isolation.

Meanwhile, Trump will cap the four-day Republican National Convention in Milwaukee with his first public address since he survived an assassination attempt in Pennsylvania on Saturday, in which a bullet grazed his ear.

The convention has put Republican unity on display in contrast to the divisions roiling Democrats. Trump's former top rivals for the nomination, former U.N. Ambassador Nikki Haley and Florida Governor Ron DeSantis, offered strong endorsements of his candidacy despite their past criticisms.

Senator J.D. Vance, Trump's running mate and another former critic-turned-loyalist, presented himself on Wednesday as the son of a neglected industrial Ohio town who will fight for the working class if elected in November.

In chronicling his hardscrabble journey from a difficult childhood to the U.S. Marines, Yale Law School, venture capitalism and the U.S. Senate, Vance, 39, introduced himself to Americans while using his story to argue he understands their everyday struggles.

"I grew up in Middletown, Ohio, a small town where people spoke their minds, built with their hands and loved their God, their family, their community and their country with their whole hearts," Vance said. "But it was also a place that had been cast aside and forgotten by America's ruling class in Washington."

As the first millennial on a major party ticket, Vance, who has embraced Trump's mixture of conservative populism and isolationist foreign policy, is well positioned to be the future leader of the Make America Great Movement.

In a sign of his potential value to the ticket, he also appealed to the working and middle classes in Michigan, Pennsylvania and Wisconsin specifically - three Rust Belt swing states likely to decide the Nov. 5 election.

Vance's prime-time debut, less than two years after assuming his first public office, caps a meteoric rise. He is one of several high-profile Republicans, such as U.S. Senators Ted Cruz and Marco Rubio, whose transformations from critics to loyalists have underscored Trump's takeover of the party.

For Trump's political opponents, his hold on the party portends a darker moment in which he follows through on his promises to expand the power of the presidency, exact revenge on his enemies and threaten longstanding democratic institutions.

Vance would advance "an agenda that puts extremism and the ultra wealthy over our democracy," the Biden campaign said on Wednesday.

Vance has opposed military aid for Ukraine and defended Trump's attempts to overturn his 2020 election loss to Biden.

His speech embraced many of Trumpism's core tenets, promising to prioritize domestic manufacturing over Chinese imports and warning allies they would no longer get "free rides" in securing world peace.

The evening's other speakers often engaged in vitriolic attacks against Biden, in contrast to the tone of national unity that Trump had promised in the wake of the shooting.

(Reporting by Joseph Ax, Nathan Layne and Gram Slattery in Milwaukee; Additional reporting by Helen Coster, Costas Pitas and Alexandra Ulmer; Writing by Joseph Ax; Editing by Howard Goller)

By Joseph Ax