Donald Trump chose Ohio U.S. Senator J.D. Vance to be his vice presidential running mate, as the Republican Party officially nominated the former president to run again for the White House on Monday at the start of the party's national convention in Milwaukee.

"As Vice President, J.D. will continue to fight for our Constitution, stand with our Troops, and will do everything he can to help me MAKE AMERICA GREAT AGAIN," Trump wrote on his Truth Social platform.

The four-day convention opened in downtown Milwaukee's Fiserv Forum two days after Trump narrowly survived an assassination attempt in Pennsylvania, and hours after he secured a major legal victory when a federal judge dismissed one of Trump's criminal prosecutions.

Trump is due to formally accept the party's nomination in a prime-time speech on Thursday and will challenge Democratic President Joe Biden in the Nov. 5 election.

Vance, 39, was a fierce Trump critic in 2016 but has since become one of the former president's staunchest defenders, embracing his false claims that the 2020 election was marred by widespread fraud.

Soon after Trump's announcement, Vance emerged on the convention floor with his wife Usha, shaking hands with and hugging delegates who swarmed the couple. He smiled widely as he was formally nominated to be vice president and is scheduled to address the convention on Wednesday.

Vance is deeply popular with Trump's core supporters, but it remains to be seen whether he can broaden the ticket's appeal. He shares Trump's aggressive approach to politics, and his conservative statements on issues such as abortion could turn off moderate voters.

Biden told reporters at Joint Base Andrews in Maryland that Vance is "a clone of Trump on the issues."

Opinion polls show a close race between Trump, 78, and Biden, 81, though Trump leads in several swing states that are likely to decide the election. Trump has not committed to accepting the results of the election if he loses.

The head of the main fundraising super PAC supporting Trump's campaign, Taylor Budowich, said on X that MAGA Inc had raised more than $50 million on Monday.

After the assassination attempt, Trump said he was revising his acceptance speech to emphasize national unity, rather than highlight his differences with Biden.

"This is a chance to bring the whole country, even the whole world, together. The speech will be a lot different, a lot different than it would've been two days ago," Trump told the Washington Examiner.

U.S. District Judge Aileen Cannon's decision on Monday to throw out federal charges against Trump for retaining classified documents after leaving the White House was the latest in a string of legal wins for the former president, who is due to be sentenced in New York in September for trying to cover up a hush money payment to porn star Stormy Daniels in the weeks before his 2016 election victory.

His other two indictments on federal charges in Washington and state charges in Georgia - both related to his efforts to overturn his 2020 election defeat - are mired in delays and could be significantly limited after the U.S. Supreme Court ruled in July that he had immunity for many of his official acts as president.

"This dismissal of the Lawless Indictment in Florida should be just the first step, followed quickly by the dismissal of ALL the Witch Hunts," Trump said on Truth Social on Monday, also referencing the prosecutions of hundreds of his supporters who stormed the U.S. Capitol on Jan. 6, 2021.