The Sanders campaign has been intensifying its attacks on Mr. Biden’s long record on Social Security, a program that Mr. Biden said Saturday he had been a “gigantic supporter” of “from the beginning.” Throughout his decades in public life, Mr. Biden has at times supported freezes and backed proposals that have alarmed some who worry about the effect on the program. He has also released a plan that calls for strengthening Social Security, and has embraced proposals like providing a higher benefit to the oldest beneficiaries.

“Joe Biden should be honest with voters and stop trying to doctor his own public record of consistently and repeatedly trying to cut Social Security,” Faiz Shakir, Mr. Sanders’s campaign manager, said in a statement on Saturday.

The Sanders campaign sees the issue as a way for its candidate to appeal to key demographic groups including older voters, particularly African-Americans — Mr. Biden’s strongest constituency — and women. Yet after days of his campaign previewing attacks on the issue, Mr. Sanders, of Vermont, himself did not press it at Tuesday night’s debate.

Both campaigns are competing aggressively in Iowa.

Mr. Biden’s campaign events here on Friday were canceled amid a snowstorm, but he spent Saturday at an education event as well as campaigning here in Indianola, while some of his top financial supporters came to Des Moines this weekend to receive briefings from campaign staff. Some braved the snow to knock on doors to promote Mr. Biden’s campaign.

On Sunday, Mr. Sanders earned the endorsement of Representative Pramila Jayapal, a rising Democrat from Washington State.

“Bernie has the bold passion, authenticity & clarity that working people across this country desperately need,” she wrote on Twitter. “We are building the progressive movement that will bring justice & opportunity & transform our country. Join us!”

Thomas Kaplan contributed reporting from Washington, and Sydney Ember from New York.