“Here in the heartland, we like our politicians in the mainstream, and he is not — he’s a socialist,” said Gov. Jay Nixon of Missouri, who is term-limited and working to elect a Democratic successor. “He’s entitled to his positions, and it’s a big-tent party, but as far as having him at the top of the ticket, it would be a meltdown all the way down the ballot.”…

“The Republicans won’t touch him because they can’t wait to run an ad with a hammer and sickle,” said Senator Claire McCaskill of Missouri, a supporter of Mrs. Clinton’s.

Mr. Sanders and his supporters deny that he would be a drag on other Democratic candidates. And Tad Devine, Mr. Sanders’s consultant, recalled another election when Mrs. Clinton’s supporters said her primary opponent was unelectable. “This is all so reminiscent about what they said about Obama in 2008,” he said.

Instead, he framed the primary as a choice between a candidate who is inspiring younger, new and independent voters or one running a 1990s-era race, focused on suburban swing voters, seizing the middle ground on issues like taxes and pushing “incremental change.”