Sen. Bernie Sanders Bernie SandersKenosha will be a good bellwether in 2020 Biden's fiscal program: What is the likely market impact? McConnell accuses Democrats of sowing division by 'downplaying progress' on election security MORE (I-Vt.) is poised to use Sunday’s Democratic debate to push former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE to the left on key policy issues as he seeks to wield his remaining political influence to shape the ideological direction of the presidential race.

With his path to the Democratic nomination looking increasingly narrow, and Biden on the cusp of building an insurmountable delegate lead in the primary race, the debate could be Sanders’s last best chance to score at least a few concessions from the former vice president on the policy issues that have defined his campaign.

“It’s a last ditch effort to see if they can turn the candidacy around,” Mark Longabaugh, a senior adviser to Sanders’s 2016 presidential campaign, said. “He wants to go out with a big discussion of his ideas.”

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Speaking to reporters in Burlington, Vt., on Wednesday, the Vermont senator telegraphed his strategy for the debate, challenging Biden on how he plans to address issues like climate change, health care and college affordability.

“Let me be very frank as to the questions that I will be asking Joe,” Sanders said, before ticking off a long list of policy issues and addressing Biden directly.

“Joe, what are you going to do to end the absurdity of the United States of America being the only major country on earth where health care is not a human right?” he asked. “Are you really going to veto a 'Medicare for All' bill if it is passed in Congress?”

An effort by Sanders to nudge Biden toward more progressive policy positions during the debate would put the former vice president in a delicate position.

He has a clear path to the nomination after racking up a series of primary wins in recent weeks. That leaves him with the task of uniting his faction of moderate Democrats with the progressive wing led by Sanders.

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“He has to reach out a hand to the progressive wing of the party that Bernie represents and figure out how to bring them into the tent,” Longabaugh said. “The party only succeeds when we’re mobilizing all the key constituencies of the party.”

Acquiescing to more liberal positions could help him assuage Sanders’s supporters, whose backing he will need in a general election bid against President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE. But doing so could also fuel attacks from Trump, who has eagerly sought to cast his Democratic rivals as radical leftists and socialists.

Longabaugh noted that some of Sanders’s policy proposals – like free college tuition – are, at least in part, popular among large swaths of the electorate, saying that Biden would be smart to bring some of those ideas into his campaign.

He recalled how Sanders and the 2016 Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE teamed up during the general election to promote her plan for debt-free college.

“There are ways in which Biden can reach out to some of the policy ideas of Sanders without enacting them in total,” he said. “I think that’s something that you’ll see and would be wise.”

Sanders’s allies insist that his fate in the presidential race isn’t yet sealed. More than half of delegates are still up for grabs in the coming weeks and he still possesses a fundraising juggernaut that has made money a non-issue for his campaign.

But Sanders’s path to the nomination narrowed significantly after a series of disappointing losses on Super Tuesday and on Tuesday’s primaries, including in Michigan, where the Vermont senator had hoped to revive his campaign with a decisive victory.

Biden, meanwhile, is riding a wave of momentum after scoring more than a dozen wins in the recent nominating contests. He is hoping to come out of next Tuesday’s primaries in Florida, Arizona, Illinois and Ohio with a near-insurmountable delegate lead that would effectively secure his spot as the Democratic nominee.

In his remarks on Wednesday, Sanders delivered a sober assessment of his campaign and the challenges ahead, acknowledging that he took a major hit with his defeat in Michigan and was losing the “debate over electability” to Biden.

“Last night, obviously, was not a good night for our campaign from a delegate point of view,” he said. “We lost in the largest state up for grabs yesterday, the state of Michigan. We lost in Mississippi, Missouri and Idaho.”

But Sanders said he was winning both the “ideological” and “generational debate” in the primary, arguing that his message of sweeping political and economic change and advocacy for far-reaching policy proposals like Medicare for All and tuition-free college spoke to the younger voters who will determine the future of the Democratic Party.

“Today, I say to the Democratic establishment, in order to win in the future, you need to win the voters who represent the future of our country, and you must speak to the issues of concern to them,” he said.

On Sunday, Sanders is hoping to elevate those issues. But he also signaled that he will look to bolster his case that he is the best candidate to take on Trump in November, saying on Wednesday that the debate – the first of the primary race to feature only two candidates – will give voters a chance to see whether he or Biden is better equipped.

“Donald Trump must be defeated, and I will do everything in my power to make that happen,” he said. “On Sunday night, in the first one-on-one debate of this campaign, the American people will have the opportunity to see which candidate is best positioned to accomplish that goal.”