He and Mr. Walsh have also been making their case to voters in one-on-one conversations in bars and coffee shops in Des Moines in the lead-up to Monday’s caucuses. James Marren, who showed up at the Exile bar to talk to Mr. Walsh the other night, said that he had encountered the candidate before, at a veterans’ group in Dubuque, and that he was a changed man from his days as a right-wing, pro-Trump radio host spouting incendiary conspiracy theories.

“It was all Democrats and him,” said Mr. Marren, 75, a Democrat so impressed by Mr. Walsh that he is talking him up to his Republican friends. “He said that Trump’s a liar and a traitor. He apologized for what he used to say on his radio show. He was amazing.”

But at the Trump rally, it was a different story. Virtually everyone Mr. Walsh spoke to disagreed with virtually everything he said.

When he mentioned Ukraine, a Trump supporter said that “Obama got that started.” When Mr. Walsh pointed out that Mr. Trump had vowed to make Mexico pay for the border wall, but was instead using money from the United States military budget, another supporter first said that Mexico was indeed paying for it, in that there are fewer Mexicans crossing the border and thus fewer Mexicans sending “American money” back home. Then she said: “Well, he would have made the Mexican government pay for it, but a lot of people are standing in his way.”

After just a few minutes of these encounters, Mr. Walsh was beginning to seem like the most masochistic person in Iowa.

“These are the people who listened to me on the radio” when he was a darling of the right, he said. “I can relate to why they voted for Trump. I voted for him in 2016. But then I saw that he lies damn near every time he opens his mouth.”

The turning point for him was Mr. Trump’s 2018 meeting in Helsinki with Vladimir V. Putin, the Russian president, in which Mr. Trump said he believed Mr. Putin’s claim — contrary to American intelligence — that Russia had not interfered in the 2016 election. “That was the most unpatriotic thing I’ve ever seen an American president do,” Mr. Walsh said.