FAIRFIELD, Iowa — Joseph R. Biden Jr. backtracked on Saturday from his stated position that he would not comply with a subpoena to testify in President Trump’s impeachment trial in the Senate. Instead, he declared that he would abide by “any subpoena that was sent to me” even as he insisted there was no justification for calling him as a witness.

A day after reaffirming that he would not comply with a subpoena, Mr. Biden tried twice on Saturday to clarify his remarks, asserting that there would be no “legal basis” for such a subpoena but left it unclear, for much of the day, if he would ultimately comply with one. Then, questioned by a voter about the issue of compliance with subpoenas, Mr. Biden answered unequivocally.

“I would obey any subpoena that was sent to me,” he said at a town hall-style event in Fairfield.

Mr. Biden’s 180-degree turn on whether he would comply with a subpoena was one of the starkest and swiftest reversals by a candidate in the Democratic primary campaign, and came after he faced questions and criticism about whether his initial stand would run counter to the rule of law.

Mr. Biden’s varied responses to a hypothetical question, which he criticized himself for drawing attention to in the first place, played out from a series of tweets on Saturday morning to remarks to reporters early in the afternoon to his answer in Fairfield on Saturday night.