Mr. Biden responded to Ms. Ernst’s comments head-on, presenting them as proof that Mr. Trump’s team was using the impeachment trial to try to hobble his candidacy.

“She spilled the beans,” Mr. Biden told the crowd, speaking in a room overlooking the Mississippi River on a gray winter day. “She just came out and flat said it. You know, the whole impeachment trial for Trump is just a political hit job to try to smear me, because he is scared to death to run against me, and he has good reason to be concerned.”

By late afternoon Tuesday, his team had sent a fund-raising email from his campaign manager with the subject line “Hey Senator Joni Ernst, let me answer your question directly.” The campaign also released a digital ad highlighting Ms. Ernst’s comments and accusing Republicans of spreading “lies” about Mr. Biden.

The impeachment trial of Mr. Trump has paralyzed Washington and frozen much of the fluid 2020 Democratic field in amber, commanding nonstop news media attention and tethering the four senators in the race to Capitol Hill. But talk of impeachment has been largely absent from the campaign trail in Iowa in the final stretch before the state’s caucuses, including at Mr. Biden’s events.

Ms. Ernst was first elected to the Senate in 2014 in a victory that helped the Republicans take control of the chamber. As she gears up for her first re-election battle this year, she has the built-in advantage of running on the same ticket as Mr. Trump, who carried Iowa in 2016 by nearly 10 points, even after Barack Obama won the state twice.