Former Vice President Joseph R. Biden Jr., they say, is Mr. Bush, pointing to his famous political legacy, deep ties to the party establishment and the sense of continuity and comfort he instills. Senator Elizabeth Warren of Massachusetts is Senator Ted Cruz of Texas: ideologically irreproachable, as far as her party’s base is concerned, but encountering some difficulty attracting a broader coalition. Pete Buttigieg, the former mayor of South Bend, Ind., is Senator Marco Rubio of Florida, the telegenic, overachieving optimist who represented the youthful ideal for mostly older voters. And Ms. Klobuchar is John Kasich, the former governor of Ohio who won only a single state — his own — but resisted calls to drop out of the race until May.

“I don’t care, pick any one of them, Biden, Klobuchar, Buttigieg or even Warren,” said Rick Tyler, a former senior strategist for Mr. Cruz. “Consolidate the vote and you’ll beat Bernie 60-40. But they’re not going to do it because the parties are so pathetic and inept and weak. We say we love primaries because they’re inclusive. Well, this is what happens.”

The debates this year are providing a similar kind of nationally televised spectacle in which the candidates rip into each other, albeit without the insinuations of sexual prowess and hyper-masculinity that the Republican debates featured four years ago.

If the general election ends up being a matchup between Mr. Sanders and Mr. Trump, the nominee of each major party would be someone who had spent much of his life unaffiliated with that party, but who has since forged a deep and somewhat unshakable connection with its base.

Mr. Sanders, who describes himself as a democratic socialist, does not formally register with the Democratic Party except to run for office. In the Senate he is an independent. Mr. Trump also switched his party registration numerous times over the last 30 years — from Republican to third party to Democratic to Republican to no affiliation to, finally, Republican again in 2012.

Mr. Sanders has not yet demonstrated a majority of support in the primary, either in the three states that have voted so far or in national polls of the race. Mr. Trump, who picked up about one-third of the votes in the early states, did not start winning outright majorities until the end of April 2016. (He, like Mr. Sanders, came close in the Nevada caucuses. Both won about 46 percent of the vote.)

Mr. Sanders is being openly attacked by his rivals, who say nominating him would amount to a gamble Democrats cannot afford to make in an election in which defeating Mr. Trump is of paramount importance. Republicans said the same thing about Mr. Trump when they similarly doubted he would be able to beat Hillary Clinton.