FARGO, N.D. — After a disappointing sixth-place finish in Nevada, which followed a less disappointing third-place finish in New Hampshire but a humbling fifth-place finish in Iowa, Senator Amy Klobuchar of Minnesota looked out at the crowd of more than 1,000 and predicted victory in the North Dakota primary — at the time, more than two weeks away.
“Somehow, I have a feeling this primary is still going to be going on,” she said.
That much seems certain.
The no-end-in-sight nature of the contest for the Democratic nomination is alarming those in the party who are hoping to blunt the momentum of the front-runner, Senator Bernie Sanders of Vermont. The most likely way they believe that could happen — a critical mass of the senator’s rivals drop out so voters can coalesce around a single alternative — seems like the least likely outcome.
The irony is thick. Mr. Sanders, the candidate many establishment Democrats fear would have the most trouble beating President Trump in November, is benefiting from some of the same dynamics that helped Mr. Trump stampede to the Republican nomination four years ago.
“It appears to me that the Democratic Party didn’t learn a whole lot from watching the Republican Party’s primary in 2015 to 2016,” said Danny Diaz, a senior adviser to Jeb Bush, the former Florida governor who went from being the early favorite to win the Republican nomination with $100 million in super PAC money behind him to dropping out less than a month into voting after he failed to win a single state.
“The reality is there’s a diffuse field that’s allowing for the establishment lane’s vote share to be diluted, and in the process allowing someone with around a quarter to a third of the vote to dominate,” Mr. Diaz said, describing Mr. Sanders’s rivals’ conundrum as if he were describing Mr. Bush’s four years ago.
As Republicans who weathered 2016 observe 2020, they are feeling a distinct sense of déjà vu. The parallels are not perfect, but there are many — right down to the candidates themselves.
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.
It is entirely plausible that Mr. Sanders could soon be on the path to winning a plurality of delegates, even if a majority of Democratic primary voters continue voting for other candidates, just as the case was for Mr. Trump after Super Tuesday.
“Just look at the Nevada caucus — he didn’t have that many votes,” said John Weaver, a former senior strategist for Mr. Kasich. “But come South Carolina and Super Tuesday, even if he gets one-third, and everyone else gets 18 percent or less, he keeps winning.”
And few of Mr. Sanders’s rivals can point to many states where they expect to win and pick up a meaningful number of delegates. (In Ms. Klobuchar’s vision of triumph in North Dakota, for example, there are only 14 delegates up for grabs, tying it with Wyoming for the lowest number awarded by any state.)
In recent days, as Mr. Sanders has opened up a wider lead, Republican strategists like Mr. Weaver have been unable to repress their amazement at the similarities.
“Been there, done that. She will be stronger politically if she drops out now versus staying in and losing till the last rat dies,” Mr. Weaver said in a tweet this week directed at Ms. Klobuchar that he has since deleted. The comparisons between Ms. Klobuchar and Mr. Kasich strike supporters of both politicians as unflattering.
As Mr. Trump continued to plow through states, Mr. Cruz, Mr. Kasich, Mr. Bush and Mr. Rubio all scrapped for seconds, thirds and fourths, splitting up the more traditional Republican vote and aiding Mr. Trump in his rise.
In making his case for the Democratic nomination, former Mayor Michael R. Bloomberg of New York has argued that the muddle of candidates vying for the support of moderate and independent voters is doing the same thing, and that they should drop out and give him a clearer path forward — a suggestion that was met with widespread disdain.
Then, as is the case now, there is hypothetical talk of how stopping the front-runner could be accomplished if the lower-polling candidates quit the race and endorsed one alternative. That kind of unanimity, the thinking goes, would make it easier for an anti-Sanders coalition to form.
It never happened in 2016, despite some false starts and half-baked attempts.
Conservatives urged Mr. Rubio and Mr. Cruz to form a joint ticket, though neither wanted to play second fiddle to the other as the vice-presidential pick.
Then Mr. Cruz announced shortly before dropping out that if he were nominated, he would choose as his running mate Carly Fiorina, the former Hewlett-Packard chief executive who had also sought the nomination. The two traveled the country together as a team.
But the combination of inertia and ego inside presidential campaigns usually stops these kinds of arrangements.
“In order to have an impact with your endorsement, you have to have support,” said Todd Harris, an adviser to Mr. Rubio’s presidential campaign. “And if you still have support, why are you dropping out?”
All the speculative talk about what a so-called unity candidate might do to knock Mr. Sanders off the launching pad assumes that that is possible in the first place, which may not be the case. Republicans still argue about whether they could have done anything to stop Mr. Trump.
Mr. Diaz, the former Bush adviser, said that Democrats should have their “eyes wide open that they’re allowing someone who has a plurality — and not a majority — to kind of walk away with the nomination.”
Mr. Tyler said that after Mr. Cruz ended his campaign, he convened a group of his former campaign aides for an informal autopsy of the race. When the senator asked, in effect, if they did it all over again what could they do to deny Mr. Trump the nomination, Mr. Tyler said his answer was nothing.
“We, like everybody else, sort of misunderstood what direction the country was going in,” Mr. Tyler said.
The data the 2016 campaigns possessed at the time suggested Mr. Trump might have always had a lock on the nomination.
At the beginning of the primary season, the Rubio campaign asked likely Republican primary voters for their preference among three potential kinds of candidates: a fiscal and social conservative who shared their values; a proven executive leader; or an outsider who wanted to shake up the system.
The outsider won by the biggest margin, with 46 percent. The values candidate won only 33 percent.
“We’ve seen this movie before,” said Whit Ayres, a Republican pollster who worked on the Rubio campaign. “The outsider runs away with the nomination.”