His path to the nomination is complicated by multi-state losses on Tuesday.

On the heels of disappointing losses during Tuesday’s primaries, Vermont Sen. Bernie Sanders weighed in on the way forward for his campaign saying that he looks forward to debating former Vice President Joe Biden.

“On Sunday, I very much look forward to the debate in Arizona with my friend, Joe Biden. And let me be very frank as to the questions that I will be asking Joe,” Sanders said on Wednesday before ticking off a list of issues that he will hold Biden accountable for addressing — progressive values that have won the Vermont lawmaker a deep bench of supporters.

Issues such as childhood poverty, climate change and economically supporting the working class are important, Sanders said, to keep at the forefront of any serious bid for office.

Sanders is the chief proponent of Medicare for All— his signature plan which would establish a single-payer system and eliminate private insurance. In June, Sanders unveiled a plan to cancel $1.6 trillion in student loan debt and to make all public colleges and trade schools tuition free and he is also a proponent of raising the federal minimum wage to $15 per hour.

Sanders’ comments come after a night in which the progressive standard bearer faced losses in Michigan, Mississippi, Missouri and Idaho–defeats which further complicate his path to netting enough delegates to net the Democratic nomination for the presidency.

The self-described Democratic Socialist indicated that the math needed to net the nomination looks difficult.

“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.”

On Tuesday night, New York Rep. Alexandria Ocasio-Cortez, a prominent Sanders surrogate and member of the “Squad”, took to Instagram Live to react to the night’s primary results. She acknowledged how difficult the evening was for Sanders and for the progressive revolution he seeks to inspire.

“You know, there’s no sugar coating it – tonight’s a tough night,” she said. “Tonight’s a tough night for the movement overall.”

But as he and his supporters acknowledge those losses, Sanders also issued a stern warning to the Democratic establishment.

“You cannot simply be satisfied by winning the votes of people who are older. While our campaign has won the ideological debate, we are losing the debate over electability. I cannot tell you how many people our campaign has spoken to who have said — and I quote — ‘I like what your campaign stands for. I agree with what your campaign stands for. But I’m going to vote for Joe Biden because I think Joe is the best candidate to defeat Donald Trump.’ End of quote,” he said. “We have heard that statement all over this country. Needless to say, I strongly disagree with that assertion, but that is what millions of Democrats and independents today believe.”

Sanders, whose strategy relied heavily on turnout, ultimately lost in Michigan, a delegate-rich state he took in 2016. Exit poll data showed that 60% of those polled in Missouri, 51% in same-day voter results in Michigan and 46% in preliminary data from Washington picked Biden as more trusted to handle a crisis.

Fifty-seven to 66% also saw him as better able to defeat Donald Trump, cementing the claim to perceived electability that lifted Biden on Super Tuesday.

Sanders himself during the start and end of his remarks on Wednesday stressed that beating Trump is the ultimate and most important goal for Democrats and those who seek a change in the Oval Office.

“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. ”