In last-minute rallies, pamphlets and television advertising and even in electric signs placed atop cars that resemble pizza delivery advertisements, the candidates are running more explicitly on their purported viability than in any modern presidential primary.

Mr. Sanders, the leading progressive in the field, is increasingly linking his populist platform to an argument that he can peel away disaffected voters from Mr. Trump while a more moderate candidate like former Vice President Joseph R. Biden Jr. could imperil that effort. Mr. Biden is closing his Iowa campaign with a commercial highlighting his advantage over Mr. Trump in some national and swing-state polls while his leading supporters in Iowa are introducing him to voters by arguing that he could win over their Republican friends.

And Mr. Buttigieg and Ms. Warren, who are struggling to match the appeal of their rivals on the party’s ideological poles, are infusing their 11th-hour messaging with barely veiled claims that they are safer choices because they won’t alienate one flank of the party like Mr. Sanders and Mr. Biden.

Returning to the Iowa campaign trail on Saturday, Ms. Warren presented herself as the candidate best prepared to rally Democrats for the general election. She addressed a group of volunteers in Urbandale, before a wall of signs reading “Unite the Party.” In a Cedar Rapids gymnasium, Representative Ayanna Pressley of Massachusetts praised Ms. Warren as “both empathetic and electable.”

“We will, we must, come together as a party and beat Donald Trump,” Ms. Warren said. “And I’ve got a plan for that.”

This turn toward I-can-win pragmatism reflects a simple fact of political life in a contest in which at least a third of likely caucusgoers remain undecided just days before the vote. The candidates are responding to the marketplace: Their voters, while deeply split across ideological and generational lines in ways that could still force a showdown at this summer’s convention, are united by an all-consuming hunger to unseat Mr. Trump.

“What I’m focused on most is getting that fool out of the White House,” said Debbie McAllister, who attended a Sioux City rally Friday for Mr. Buttigieg, the former mayor of South Bend, Ind., and is leaning toward him over Ms. Klobuchar.