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’s third run for the White House will test whether an old-school politician from another era can reinvent himself for modern times more than 30 years after he made his first bid for the White House.

Biden enters the race for the Democratic presidential nomination as the front-runner, the best-known and most beloved figure in a crowded contest that has exposed a party in search of a unifying identity.

He leads every national poll of potential Democratic primary voters, and he runs ahead of or just behind 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.) in early state match-ups.

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But Biden’s road to the Democratic nomination is beset by both strategic and ideological hurdles as he vies to lead a party that has moved to the left since he was last on the ballot in 2012.

The contrasts are everywhere.

Biden’s main rivals are building war chests with armies of small-dollar donors, while Biden’s first campaign event will be a high-dollar fundraiser at the home of a wealthy lobbyist.

While many of those he will run against are staking out positions central to the Democratic Party’s growing liberal base, Biden has spent weeks revisiting some of his more controversial, and conservative, positions in the past.

He has expressed remorse for the way he handled the Anita Hill hearings, and for a crime bill he backed in 1994 that led to disproportionately high rates of incarceration for young black men.

More recently, he acknowledged that “social norms have begun to change” after several women said he had made them uncomfortable in the course of showing affection or offering support.

Biden will pitch himself to Democratic voters desperate to end the Trump presidency as a steady, experienced hand with the best chance to beat the sitting president.

His video announcement released Thursday morning set up the most direct challenge to 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 of any of the candidates who have announced so far. Over a montage of Martin Luther King Jr., suffragettes and American troops landing at Normandy, Biden said Trump would be viewed as “an aberrant moment in time.”

“The core values of this nation, our standing in the world, our very democracy, everything that has made America America is at stake,” Biden said.

His message sounded, in part, like an echo of Trump’s own pledge to make America great again.

“America’s coming back like we used to be,” Biden told reporters in Wilmington, Del., on Thursday afternoon, hours after launching his bid.

Biden is hoping to buck history. Sitting and former vice presidents have struggled to win the presidency in modern politics, in part because voters tire of one administration.

Since World War II, only two vice presidents — Richard Nixon, eight years after he lost the presidency, and George H.W. Bush, succeeding Ronald Reagan — have won the presidency on their own.

Three others — Walter Mondale, Al Gore Albert (Al) Arnold GoreBusiness groups start gaming out a Biden administration Cruz says Senate Republicans likely have votes to confirm Trump Supreme Court nominee 4 inconclusive Electoral College results that challenged our democracy MORE and Hubert Humphrey — have won the Democratic nomination but lost to Republicans in the general election. Dan Quayle’s campaign in 2000 did not even make it to Iowa’s first-in-the-nation caucuses.

Biden begins his bid in a strong position, albeit not as strong as Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE did four years ago. But the front-runner’s crown sits precariously on his head, after he got a late start and staked out a more centrist role than his most significant rivals.

If Democratic voters see Biden as the course to a safe harbor, his best days may be ahead. If they opt to steer toward the future, his best day may be today.