WASHINGTON (AP) — In April, Bernie Sanders repeatedly stood shoulder to shoulder with President Joe Biden, promoting their joint accomplishments on health care and climate at formal White House events while eviscerating Donald Trump in a widely viewed campaign TikTok video.

Then just last week, Sanders was bluntly warning that the crisis in Gaza could be Biden's "Vietnam" and invoking President Lyndon B. Johnson's decision not to run for reelection as the nation was in an uproar over his support of that war.

Such is the political dichotomy of Bernie Sanders when it comes to Joe Biden. They are two octogenarians who share a bond that was forged through a hard-fought primary in 2020 and fortified through policy achievements over the last three years.

Now, in this election year, Sanders will be Biden's most powerful emissary to progressives and younger voters — a task that will test the senator's pull with the sectors of the Democratic Party most disillusioned with the president and his policies, especially on Gaza.

Privately, Sanders has felt less enthusiastic in recent days about making the political case on Biden's behalf as the Gaza crisis worsened, according to a person familiar with Sanders' sentiments. Still, Sanders remains adamant that the specter of Trump's return to the Oval Office is too grave a threat and stresses that "this election is not between Joe Biden and God. It is between Joe Biden and Donald Trump."

"I understand that a lot of people in this country are less than enthusiastic about Biden for a number of reasons and I get that. And I strongly disagree with him, especially on what's going on in Gaza," Sanders said in a recent interview with The Associated Press.

But Sanders continued: "You have to have a certain maturity when you deal with politics and that is yes, you can disagree with somebody. That doesn't mean you can vote for somebody else who could be the most dangerous person in American history, or not vote and allow that other guy to win."

That will be the thrust of the message that Sanders will carry through November, even as progressive furor over Biden's handling of the war in Gaza continues to escalate, protests continue to fester and Sanders' own critiques of the administration's policy become more pointed.

"He's not trimming the sails on Gaza, because of Biden," said Sen. Peter Welch, D-Vt., who succeeded Sanders in the House and joined him in the Senate last year. "Bernie's credibility is that he's maintained his solid positions, and then he's going to make the case why, Biden versus Trump."

A WHITE HOUSE-SENATE PARTNERSHIP

Few can doubt Sanders' influence throughout the Biden presidency. Once rivals for the Democratic presidential nomination in 2020, the two men later joined forces to assemble half a dozen policy task forces that underpinned the party's policy platform later that year — an unusual endeavor that helped bring the Democratic socialist's supporters into Biden's fold.

That laid the groundwork for a burst of ambitious policymaking in the first two years of the Biden administration, from a sweeping $1.9 trillion pandemic relief package in early 2021 to legislation in the summer of 2022 that was a mishmash of longstanding Democratic priorities, including cheaper prescription drugs for Medicare beneficiaries. Sanders, who helped craft those blueprints as head of the Senate Budget Committee, had been directly encouraged by Biden to go big in those proposals, with the assurance that the president had his back.

"You and I have been fighting this for 25 years," Biden told Sanders admiringly at their joint health care event in April. "Finally, finally we beat Big Pharma. Finally."

Sanders, like many others who back Biden's domestic achievements, believes the public is still too unaware of them. He was the one who approached White House officials about doing an event specifically to spotlight a drop in the cost of inhalers.

More than three years into Biden's term, Sanders' connections throughout the West Wing are deep. He chats regularly not only with the president, but his top aides, including White House chief of staff Jeff Zients, senior adviser Anita Dunn and national security adviser Jake Sullivan.

"He doesn't mince words," Dunn said. "He's very direct with us, pretty blunt, and that's a good thing."

DEEP TENSIONS OVER GAZA, CAMPAIGN STRATEGY

It took just hours for Sanders, who announced his own reelection bid Monday, to endorse Biden's campaign once the president made it official last April. It was an unmistakable signal to his supporters that, despite any misgivings, it was imperative to back Biden without hesitation.

Yet some Democrats are worried that anger among progressives over Gaza is so deep that not even Sanders can persuade them to support Biden. A persistent bloc of voters in multiple primaries continues to choose "uncommitted" or a variant to protest Biden's handling of the Israel-Hamas war, sometimes far surpassing Biden's margin of victory in those same states in the 2020 general election.

For instance, more than 48,000 voted "uninstructed" in the Wisconsin Democratic primary in early April, which outpaced the roughly 20,700 votes by which Biden outpaced Trump, a Republican, in the battleground state four years ago. Wisconsin's primaries this year came three weeks after Biden had already clinched the nomination.

"This campaign is in trouble. And Sen. Sanders will do everything — again, everything — that he can to try to pull this man over the finish line," said Nina Turner, who was a national co-chair of Sanders' 2020 campaign. "I'm not so sure it's going to work this time."

Mitch Landrieu, a national co-chair for the Biden campaign, told CNN that Sanders' comparisons to the Vietnam War were an "over-exaggeration." A March poll conducted by the Harvard Institute of Politics found that 18- to 29-year-olds were less likely to say the Israeli-Palestinian conflict was the national issue that concerned them most, compared to issues like the economy, immigration and abortion.

But it isn't just on Gaza that Sanders has been pushing Biden and his aides. He's urging them to shift campaign strategy to not just contrast Biden with Trump but to lay out ambitious goals on health care, education, child care and workers' rights.

Biden's State of the Union address, which his advisers point to as a roadmap for his second term, was a "general start," Sanders said, but he added that Biden has to do more to inspire voters.

"What I've said to the White House is, it's not good enough simply to talk about Donald Trump," Sanders said in the interview. "It's not good enough to talk about your accomplishments, which I have. You got to have a bold agenda for the future."

Biden's aides point to specific proposals released around the State of the Union, such as an expansive housing plan that would build or preserve two million homes. Sanders is also now developing new health care legislation in tandem with the White House, which would extend to all Americans the $2,000 annual cap on prescription costs that the Inflation Reduction Act provided to seniors on Medicare.

SHARED VALUES, IF NOT IDEOLOGIES

Biden doesn't hesitate to point out where he splits with Sanders when given the chance.

"I like him, but I'm not Bernie Sanders. I'm not a socialist," Biden said in January 2022. "I'm a mainstream Democrat."

Yet top advisers to the president, long a stalwart of the Democratic center-left, and Sanders, the undisputed leader of the party's progressive wing, say the two men share more traits than their ideological stances would indicate.

For one, they both hold a core belief that government should be a force for good. Their political careers are anchored in small, sparsely populated states that exposed them to the most hyperlocal and grassroots of politics. They have a sense of pragmatism about working within the political system's realities, even if Sanders works to push those boundaries and Biden governs inside of them.

Biden, as vice president, was the rare establishment Democrat who was warm to Sanders during the senator's first presidential bid. He invited Sanders to the vice presidential residence at the Naval Observatory to discuss his campaign and policy ideas in 2015 — a time when tensions between Hillary Clinton's coalition and the ascendant Sanders wing were increasingly embittered.

"I know he felt that while there was a lot of hostility within the Democratic Party and in the top ranks … he felt warmth and positivity from Joe Biden," said Faiz Shakir, who served as campaign manager for Sanders' 2020 campaign and remains a close political adviser.

Even as the 2020 debates were fiercely fought, Biden and Sanders never let the disputes turn personal. Rep. Ro Khanna, D-Calif., another national co-chair for Sanders in 2020, recalled that when some of his aides wanted to forcefully attack Biden in personal terms, the senator would respond, "Absolutely not."

'I'LL BE ACTIVE' IN 2024, SANDERS SAYS

Now, Sanders is determined to ensure Trump doesn't win again.

The Biden campaign has made it clear to Sanders' political team that they want him engaged as much as possible, seeing his longstanding connections with key voting blocs as an asset. Because Sanders campaigned for Biden four years ago, the reelection team also knows well specifically how Sanders would be most helpful for Biden.

It wouldn't be a surprise, for instance, if Sanders were again dispatched to Michigan, where he stumped for Biden in October 2020, or at union halls to energize working-class voters.

"He knows himself, his team knows him and we know what has worked," said Carla Frank, the Biden campaign's director of surrogate operations.

For his part, Sanders is still wrestling with precisely how he can be the most effective as a campaigner this fall and how he can best target the audiences that most need to hear his case for Biden, according to aides.

But "I intend to be aggressive," Sanders said.

"I see this as an enormously important election that I for one will not sit out," he added. "I'll be active."

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Associated Press writer Lisa Rathke in Marshfield, Vermont, contributed to this report.