Sen. Bernie Sanders acknowledged on Wednesday that he is trailing former Vice President Joe Biden, but said he will stay in the race and will debate Biden on Sunday.

Sanders also acknowledged that he is struggling to persuade voters that he is the best candidate to defeat President Donald Trump in November. However, he said his campaign was “winning the generational debate,” with broad appeal among younger voters.

Biden won four of the six states up for grabs on Tuesday night, including Michigan, where Sanders had defeated Hillary Clinton four years ago. Sanders won in North Dakota, and was narrowly leading in Washington.

Biden’s victories put him on a glide path to the Democratic nomination, and have spurred calls among some Democratic insiders to bring the primary to a close.

But Sanders said he would stay in the race at least long enough to debate Biden one on one in Phoenix on Sunday. The debate will be co-hosted by CNN and Univision, and will be held without a live audience due to coronavirus.

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

Sanders said he would challenge Biden to address health care costs, medical bankruptcies and climate change, among other issues.

In remarks in Philadelphia on Tuesday night, Biden struck a theme of unity, saying it was time for Democrats to rally together to defeat Trump. He also paid tribute to the “energy” and “passion” of Sanders’ supporters.

“We share a common goal, and together we’ll defeat Donald Trump,” Biden said.

Sanders did not take questions from reporters after giving his remarks in Burlington, Vt.