A defiant Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) on Wednesday vowed to continue on in the race for the Democratic presidential nomination following a string of dispiriting losses to former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE.

Sanders and Biden will square off for their first one-on-one debate of the cycle on Sunday in Arizona, and Sanders said he’d challenge Biden on a host of issues there.

The Vermont progressive acknowledged that he’s losing in the fight for delegates, saying that Democrats are worried that he’s not as electable as Biden. But Sanders is pushing on, saying he'd make the case for his progressive politics even as his losses mounted.

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“We have won the ideological debate, but we are losing the debate over electability,” Sanders said from Burlington, Vt. “I cannot tell you how many people our campaign has spoken to who say they agree with us but will vote for Joe because they believe he’s the best to beat Donald Trump. Needless to say, I strongly disagree with that assertion, but that’s what millions of Democrats and independents say. On Sunday, I very much look forward to the debate.”

Biden won big victories on Tuesday night in Michigan, Missouri, Mississippi and Idaho, growing his delegate lead to about 150 over Sanders.

The race is not over, and Biden is still only about halfway to the 1,991 delegates a candidate needs to win the nomination. But the proportional allocation of delegates makes it difficult to overcome the kind of deficit Sanders faces.

In addition, polls show Sanders trailing by huge margins in Florida and Arizona, which will vote on Tuesday. And Biden is looking at a big victory in Georgia on March 17, where he’s expected to continue his run of strong showings in the South, where black voters have turned out in huge numbers for his campaign.

Pressure is mounting on Sanders to either drop out or to rein in his attacks on Biden, who most expect will be the nominee against President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE in November.

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Rep. James Clyburn (D-S.C.), whose endorsement of Biden ahead of the South Carolina primary was a turning point in the race, has said the Sunday debate should be canceled.

Several top Democratic super PACs have declared Biden the presumptive nominee and have begun refocusing their efforts on helping him win the general election in the fall.

Importantly, Sanders on Wednesday did not frame his opposition to Biden as attacks against his record, instead asking over and over again “Joe, what are you going to do?” to address progressive concerns, ranging from wealth inequality to health care and immigration.

Sanders also reiterated that his overriding commitment is to ensuring that Trump is not reelected in November, perhaps a signal to Democrats that he does not intend to do anything that would harm the party or the likely nominee.

“Donald Trump must be defeated, and I will do everything in my power to make that happen,” Sanders 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.”