Virtually the entire Democratic presidential primary has been predicated on one underlying assumption: the eventual collapse of Joe Biden.

More than a dozen of his erstwhile opponents bet their campaigns on it, and he’s proven them all wrong. But his last serious opponent standing, Bernie Sanders, is still clinging to the same belief — and that it will finally happen on Sunday night, under the bright lights of their one-on-one debate.

To revive his campaign, Sanders needs to prove he’s the best candidate to take on President Donald Trump and lead the country at a time of duress, his aides and allies say. But they also acknowledge privately it’s going to take more than that — namely, for the gaffe-prone Biden to stumble.

“In the best-case scenario, Bernie dominates the debate and the arguments. He has one or two good one-liners,” said Mark Longabaugh, a former senior adviser to Sanders’ 2016 campaign. “And obviously the flip side would be Biden has a lackluster debate and maybe makes a mistake. All that coming together maybe causes some pause and hesitation on the part of voters.”