INDIANOLA, Iowa — Former Vice President Joseph R. Biden Jr. on Saturday accused Senator Bernie Sanders’s presidential campaign of distorting his record on Social Security, claiming without evidence that Mr. Sanders’s team was promoting a “doctored” video, a loaded word in an era of disinformation.

“There is a little doctored video going around,” Mr. Biden said, adding that it was “put out by one of Bernie’s people.”

The remarks, which Mr. Biden made at a campaign event here Saturday afternoon, marked a departure from his typical oblique swipes at opponents when speaking before voters, and seemed to signal an escalation in tensions between the two contenders who are competing for an overlapping slice of blue-collar Iowans here in the leadoff caucus state.

The Sanders campaign claimed recently that Mr. Biden had praised former House Speaker Paul D. Ryan, Mr. Biden’s 2012 rival for the vice presidency, in support of making cuts to Social Security. On Jan. 1, a Sanders adviser circulated on Twitter a video clip that captured only a portion of a Biden speech that appeared to promote that argument, without context, though the video did not appear doctored.

On Saturday, Mr. Biden invoked PolitiFact, the fact-checking outlet, which ruled that the Sanders campaign claim was false and lacked context. The PolitiFact ruling did not focus on the video, contrary to Mr. Biden’s suggestion, but it did address the substance of the Biden speech at issue.

“It is simply a lie, that video that’s going around,” Mr. Biden said, adding, “I’m looking for his campaign to come forward and disown it. They haven’t done it yet.”

The remarks in Indianola came as the Sanders campaign was still dealing with a bruising clash with Senator Elizabeth Warren of Massachusetts over his campaign tactics and whether he had told her in 2018 that a woman could not be elected president.

The Sanders campaign has been intensifying its attacks on Mr. Biden’s long record on Social Security. Throughout his decades in public life, Mr. Biden has at times supported freezes and backed proposals that has 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.

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