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DES MOINES — Ron Paul was back in Iowa on Wednesday, but not to campaign. He testified as an uncooperative witness in the federal corruption trial of two former top aides from his 2012 presidential campaign, including Jesse Benton, who is married to Mr. Paul’s granddaughter.

“I didn’t know you could be compelled to testify against your own family,” Mr. Paul, a former United States representative from Texas, said on the stand. “That is a heavy burden.”

He accused the government, which brought an indictment in August the day before the first Republican debate, of political motives aimed at harming the 2016 campaign of his son, Senator Rand Paul of Kentucky. “I don’t consider that a coincidence,” he said.

Mr. Benton, who stepped down as head of a “super PAC” supporting Rand Paul, faces charges with another top Paul aide, Dimitri Kesari, relating to hidden payments to an Iowa state senator to endorse Ron Paul on the eve of the 2012 caucuses.

The case has cast a shadow over the campaign of Rand Paul, who was at an event just a few miles away in Des Moines on Wednesday, struggling to reignite his fortunes in a state where he was once seen as inheriting the deep organization and support of libertarian-minded voters that his father built four years ago.

Ron Paul described Mr. Benton as “my right-hand man” in his presidential race, a constant traveling aide and the person he turned to for communications because Mr. Paul did not use email.

Prosecutors say Mr. Benton and Mr. Kesari paid $73,000 in campaign cash to Mr. Sorenson to endorse Mr. Paul after quitting as chairman of Michele Bachmann’s campaign.

Mr. Sorenson has pleaded guilty to concealing the payments and faces 25 years in prison. He is expected to be the government’s main witness. The prosecution says the two defendants — charges against a third key former Ron Paul aide, John Tate, were thrown out by the judge — arranged to hide the payments after Mr. Sorenson told Megyn Kelly of Fox News in an interview that he had not been paid to switch teams.

In an email introduced in court, Ron Paul’s campaign manager asked for an explanation for repeated payments, and Mr. Kesari replied that they were for “the deal Jesse agreed to with Kent.”

On the stand, Mr. Paul seemed barely to know who Mr. Sorenson was. He recalled being annoyed by the suddenness of a news conference called for Mr. Sorenson to endorse him in December 2011, days before the Iowa caucuses.

He denied any knowledge of payments made for the endorsement. Asked by the defense if he felt victimized by former aides, Mr. Paul, a lifelong critic of federal overreach, grew animated. “I blame the government,” he said. “How can I be a victim of my staff?”

Mr. Paul said he took a dim view of all political endorsements, recalling a 1996 race for Congress, when Newt Gingrich, then speaker of the House, lined up endorsements for a rival in the Republican primary. Mr. Paul said his sole endorsement was by the baseball pitcher Nolan Ryan — and he won.

The former presidential candidate, a fixture four years ago in Iowa, where he finished third with 21.4 percent of the vote, brushed aside reporters as he left the courthouse.

“Just keep walking straight,” he told aides.

On East Walnut Street a jogger recognized him and shouted, “Oh man, how you doing?”

A man on a bicycle shouted, “Hey Ron!”