Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE will officially enter the presidential race on Thursday, according to a source close to his campaign.

Biden has been flirting with entering the race for months and will begin his campaign in the lead in several surveys nationally and in early-voting states.

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Biden will announce his candidacy with a video released on Thursday and then will travel to Pittsburgh for a more formal event on Monday, the source said.

Biden, who would be 78 when entering the Oval Office if he wins the presidency, faces a crowded field and a number of questions about his candidacy.

A new poll from Monmouth University released Tuesday found Biden leading Democratic candidates and winning 27 percent support nationally, 7 points higher than the 20 percent won by Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.).

Sanders may be the top competition in the race for Biden initially. A series of polls have had the two bunched together at the top, with some distance from other candidates.

Biden will face questions about whether he is liberal enough to win his party's nomination.

There have been questions already about his support for a Clinton-era crime bill and his backing for the Iraq War.

Biden also is dealing with a lingering controversy involving his touching and kissing of a number of women, several of whom said it was inappropriate.

Biden's campaign is expected to argue he is the Democrat most likely to defeat President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE in the general election.