So whether it’s with expensive advertising, sharpened attacks on the leading candidates or retrenching to make their stand in a single state, the longshots are taking their shots.

“The assessment for a number of candidates on the brink of the top tier is that one of them is bound to break into the top tier, so why can’t it be them?” said Jennifer R. Psaki, who worked as a campaign and White House aide to former President Barack Obama.

That so many are even trying reflects the unpredictable nature of this primary.

While a handful of candidates have already quit, unwilling to suffer the indignity of trudging on without making the debates, Mr. Biden’s penchant for self-inflicted errors has prompted most of those polling in single digits to believe his hold on the lead is tenuous and that they should stay in the race. And for the more centrist candidates, their calculation is that the half of the primary electorate that calls itself moderate or conservative will not turn to Mr. Sanders or Ms. Warren if and when Mr. Biden fades.

These twin wagers have meant an unusually large number of underdogs are forging ahead against the odds, ensuring that the historically large Democratic field remains unwieldy and that the lagging candidates must go to new lengths to get attention.

Senator Michael Bennet of Colorado and Gov. Steve Bullock of Montana, for example, both failed to meet the threshold for this week’s debate. But these two western moderates believe Mr. Biden will falter by Iowa — Mr. Bennet called him a “soft’’ front-runner last weekend — and that the establishment wing of the party will be scrambling for an alternative.

So they’re sticking around in expectation of that moment: Mr. Bullock is planning on a modified version of Ms. Klobuchar’s strategy, spending the bulk of his time in Iowa, and has begun spicing up his language with expletives. And Mr. Bennet turned to another time-honored publicity grab on Saturday, the big-name endorsement, when he rolled out former Senator Gary Hart at the New Hampshire Democratic convention.