But in Iowa, Democrats who hope to avert that outcome do not appear close to settling on another candidate as an alternative to Mr. Sanders. And if more moderate voters don’t coalesce behind an alternative by next week’s caucus, party traditionalists fear, Mr. Sanders could win Iowa with only a modest plurality, emboldening his leading rivals to remain in the race, and then notch another victory again a week later in New Hampshire. No Democrat in modern times has lost contested races in both Iowa and New Hampshire and claimed the nomination.

The early primary and caucus outcomes could have an outsize impact on later primaries, including the large states voting in March, some of which begin collecting mail-in and early ballots in the immediate aftermath of Iowa. If a candidate like Mr. Sanders were to seize momentum next week, it could help him build a head start in states like California and Texas.

It is a scenario that is deeply alarming to establishment-aligned Democrats, if not unfamiliar. Four years ago, convinced Donald Trump could not win the presidency, they watched with delight as he snatched the Republican nomination without winning majorities because his more traditional rivals divided the vote and refused to bow out.

The Democrats in this race have been as reluctant to target Mr. Sanders as the Republicans were to target Mr. Trump four years ago; in each case they were skeptical of his staying power and believed they had more to gain by attacking other rivals.

Even now, as Mr. Sanders takes a lead in the first two early states, his opponents have not delivered a sustained argument against his candidacy, and remain reluctant to take him on: while Mr. Buttigieg drew attention for warning in a fund-raising solicitation that a Sanders nomination would be too risky, he notably declined to amplify his rhetoric in television interviews over the weekend. The closest he has come to confronting his rival on the left is to make oblique references to the often-bitter 2016 primary between Hillary Clinton and Mr. Sanders.

“Most of us would agree the less 2020 resembles 2016 the better — in all respects,” Mr. Buttigieg said in a brief interview. Each of the would-be Stop Sanders candidates has built enough political strength to justify forging ahead: Mr. Biden remains the national front-runner, with unmatched support among black voters; Mr. Buttigieg and Ms. Warren both have double-digit support in New Hampshire polls, and sizable war chests; Ms. Klobuchar has the thinnest operation beyond Iowa of the group, but over the weekend she earned the endorsement of New Hampshire’s influential Union Leader newspaper.