“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.”

It’s a tall order: Biden has won 14 of the 20 states that have voted this month, and he’s polling about 20 to 40 percentage points ahead of Sanders in the states that cast ballots on Tuesday.

To make matters worse for Sanders, Tuesday was going to be difficult for him even in the best-case scenario: Its biggest delegate prize is Florida, home to many older, moderate Democrats and Cuban exiles who eschew his left-wing politics. It is also a closed primary where only Democrats can vote, and Sanders typically performs better in races where independents can participate.

The reality of the situation isn’t lost on Sanders’ aides. “We’re not naïve,” said Faiz Shakir, Sanders’ campaign manager. “It’s going to be a hard path.”