A defiant Sen. Bernie Sanders promised Wednesday to hammer Democratic front-runner Joe Biden over key progressive issues at Sunday’s head-to-head debate, even as Sanders admitted that a slew of primary losses has left his campaign on the ropes.

“I very much look forward to the debate in Arizona with my friend Joe Biden,” said Sanders about the forum slated for Phoenix — before detailing his promise to whack at the suddenly surg­ing Biden on a checklist of Bernie’s progressive priorities.

The Vermont lawmaker and self-described democratic socialist detailed his plan to debate Biden on health care, poverty, climate change, mass incarceration and reforming the criminal-justice system, saying “a strong majority of the American people support our progressive agenda.”

Particularly, Sanders emphasized that there are vast differences between the two over his Medicaid for All plan to replace private health insurance, which he said would help working families who at present are “paying an average of 20 percent of their incomes for health care.”

He also vowed to grill Biden on helping young people pay for college education and fixing “our broken and inhumane” immigration policies.

And in a veiled shot at media mogul Mike Bloomberg’s recently ended presidential campaign, he said: “Joe, importantly, what are you going to do to end the absurdity of billionaires buying elections and the three wealthiest people in America owning more wealth than the bottom half of our people?”

But Sanders acknowledged while speaking in his hometown of Burlington, Vt., that he faces a difficult path to the Democratic presidential nomination after Biden scored another round of key wins Tuesday, including in Michigan, where Sanders in 2016 scored an upset primary victory against Hillary Clinton.

“Last night obviously was not a good night for our campaign,” Sanders said. “From a delegate point of view, we lost in the largest state up for grabs yesterday, the state of Michigan,” with 125 delegates up for grabs.

He also noted that he lost in Mississippi, Missouri and Idaho but won in North Dakota and that Washington state was still too close to call.

He conceded that his campaign is “losing the debate over elect­ability” and failing to attract key Democratic voting blocs, including African Americans.

But he downplayed those losses while claiming his campaign is winning the “generational debate” and attracting younger voters.

But the road ahead for Sanders will get even rougher.

In delegate-rich Florida, Biden is polling ahead of Sanders and has made inroads with Latino voters there unhappy with Bernie’s praise of late Cuban dictator Fidel Castro’s efforts to increase literacy among his citizens.

Biden leads in the delegate count 857 to 709. A candidate needs 1,991 delegates to win the nomination at the Democratic National Convention in July.