WASHINGTON (AP) — Since leaving the White House nearly four years ago, former President Barack Obama has repeatedly called for a new generation of political leaders to step up. On Wednesday, he'll implore Americans to vote for Joe Biden, a 77-year-old who has been on the national political stage for more than four decades.

Obama's political support and personal affection for Biden is genuine, forged during their eight years together in the White House. The former president has been a private sounding board for Biden throughout the 2020 contest and an active public presence in the campaign since the Democratic primary ended.

Yet there's an inherent tension in Obama's role as one of the most powerful and important surrogates for Biden. Obama's own political rise was fueled by the power of barrier-breaking, generational change, and he's encouraged "new blood" in politics. More recently, Obama drew attention during the 2020 Democratic primary when he said many of the world's problems have been due to "old people, usually old men, not getting out of the way."

In a live address Wednesday at the Democrat's virtual convention, Obama will aim to serve as a bridge between those reassured by Biden's lengthy resume and more moderate record, and a younger generation of Democrats agitating for a seat at the table and pushing for more sweeping changes to the nation's economic and domestic policies.

"He is an incomparable witness for Biden," David Axelrod, a longtime Obama political adviser, said of the former president. "But he also speaks compellingly to young people restless not just to purge Trump, but to achieve real, meaningful change."

Despite their close relationship, Obama has had to walk a tightrope when it comes to Biden's presidential ambitions. During the 2016 race, Obama privately discouraged Biden from running against Hillary Clinton for the Democratic nomination. He also stridently refused to endorse a candidate in the 2020 Democratic primary, despite pressure from within the party to weigh in.

With the general election now in full swing, Obama confidants say that while the former president's support for Biden is unequivocal, he does worry about enthusiasm among younger voters, particularly younger voters of color. He's well aware that one of the reasons President Donald Trump currently occupies the Oval Office is that those voters did not show up in the same large numbers in 2016 for Hillary Clinton as they did when he was on the ballot.

Obama's wife, former first lady Michelle Obama, spoke directly to those concerns in her own well-received address on Monday's opening night of the convention, urging Democrats to "vote like we did in 2008 and 2012." The president is expected to make a similarly urgent appeal, as well as take aim at what an aide described as "the cynical moves by the current administration and the Republican Party to discourage Americans from voting."

The fact that the Obamas are headliners on two of the four nights of the Democratic convention speaks to the crucial role they have in helping Biden try to reassemble the coalition that propelled them into the White House — and the challenge the Democratic Party has in building a new bench of other leaders who can do the same.

"When you think about folks who have the capacity to really unify us, there are only a few people," said Yvette Simpson, chief executive of Democracy for America, a progressive political action committee. "Certainly Barack Obama and Michelle Obama are among them."

Indeed, the former president has enviable popularity, both among Democrats and all Americans. A Fox News poll conducted in May found 93% of Democrats had a favorable opinion of Obama, as did 63% of all registered voters.

Despite that strong support, there has been some rethinking of Obama's legacy among some of his party's most liberal activists, who argue he didn't go far enough in overhauling the nation's health care system and gave too much away to Republicans in fiscal negotiations. Obama himself has acknowledged there was more he wanted to do, but argued he was hamstrung by the realities of a Republican-controlled House, and eventually Senate, for much of his tenure.

But some of Obama's more recent comments have energized liberals, who see signs of him embracing some of the tactics of his party's activist wing. Progressives cheered in particular when Obama called for eliminating the Senate filibuster rules requiring 60 votes on major pieces of legislation, calling it a "Jim Crow relic" that is holding up rewriting voting rights laws. His surprise comments came during his eulogy at the funeral of the late civil rights leader and Georgia Rep. John Lewis.

"That's the guy we remember from the election of 2008," Simpson said of Obama's remarks at Lewis' funeral. "It encouraged me that he might be the guy that pulls Joe Biden along a little bit."

_

Follow Julie Pace at http://twitter.com/jpaceDC