After another tough night and facing a steep path to regaining a delegate lead, Sen. Bernie Sanders of Vermont insisted Wednesday that he's staying in the presidential race.

Sanders and former Vice President Joe Biden on Sunday evening are set to debate each other one-on-one for the first time.

Sanders has been adamant that he would back whichever Democrat accumulates an insurmountable delegate lead, including in the event the leader secures a plurality but not a majority to clinch the nomination.

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Sen. Bernie Sanders of Vermont on Wednesday vowed to remain in the Democratic presidential primary after suffering another setback Tuesday night, losing the key state of Michigan with his delegate deficit to former Vice President Joe Biden growing.

"Last night, obviously, was not a good night for our campaign from a delegate point of view," Sanders said.

Sanders, however, touted strong support in polling for some of his key campaign proposals, such as wealthy people and corporations paying more in taxes and raising the minimum wage to $15 an hour.

Sanders, 78, went into a version of his normal stump speech decrying inequality and "this absurd healthcare system," which he argued had been brought into stark relief amid the spread of the coronavirus.

While Sanders trails Biden by 647 delegates to 790 — a steep deficit to overcome given Biden's performance and the Democratic Party's proportional delegate allocation — he argued that his campaign's strength among young voters was a chief reason he remained in the race.

Sanders added that voters kept telling his campaign that despite agreeing with his platform, they believed Biden was more likely to beat President Donald Trump.

"Needless to say, I strongly disagree with that assertion," Sanders said.

Sanders said he would make the case on Sunday night's debate stage that he was the more electable candidate and would deliver pointed questions to Biden about his plans for the working class and on climate change and whether he would veto "Medicare for All" healthcare legislation.

The debate in Arizona will be held without a live audience because of concerns over the coronavirus, and it will be the first one-on-one encounter of the cycle.