It’s crunchtime for the Democratic presidential candidates. The second set of debates will air live from Detroit on Tuesday and Wednesday — with 20 contenders split evenly between the two nights — and the four-hour spectacle is being pitched as an elimination round worthy of “The Bachelorette.” The debates scheduled for September and October have a higher threshold for participation in terms of poll numbers and campaign donations, which means that, among the more marginal candidates, those who fail to distinguish themselves in this showdown might not get another shot.

The candidate with the most on the line isn’t a fringe player like Marianne Williamson or John Delaney. (To review: She’s a self-help guru; he’s a former congressman from Maryland. Do not feel bad if this is the first you’ve heard of them.) Rather, it’s the pack’s front-runner, Joe Biden, slated to appear on Wednesday. The former vice president’s performance in the first round of debates last month was, to put it gently, unsettling. When Senator Kamala Harris went after him on the issue of busing for school integration, Mr. Biden crumbled like a stale cracker — and never fully recovered. With his halting speech and occasionally befuddled demeanor, Mr. Biden didn’t simply leave Democrats questioning whether he can beat President Trump; people across the political spectrum were openly musing about whether he is too old to handle another White House run. When the debate clock ran out on one of his answers, prompting Mr. Biden to mumble, “My time is up. I’m sorry,” the attack ads began writing themselves.

There’s intense pressure on him to obliterate the memory of this stumble. Democrats are desperate for a winner, and Mr. Biden’s core appeal for many is the sense that he is their safest bet — not exciting, or even inspiring, but an experienced, centrist pragmatist unlikely to scare the older, whiter, more moderate, less woke voters whom the party is itching to woo back. If Mr. Biden starts to look risky, for whatever reason, he loses his competitive advantage. The question then becomes: Who can take over the slow-and-steady lane that he has been dominating?

Even strong candidates can have a weak debate showing. In 2012, President Barack Obama’s poor performance in his first matchup against Mitt Romney prompted panic in Democratic circles. Fair or not, Mr. Biden’s margin for error here is smaller. This is in part an issue of basic biology. Now 76, he would be the oldest person ever elected president. (Ronald Reagan was 73 at the start of his second term.) Mr. Biden first ran for president more than three decades ago. Questions about whether he is “slipping” are perhaps inevitable. Mr. Trump, despite his own advanced age at 73, has been his usual subtle self, suggesting that the former vice president is “losing it.”