President Trump remembered the conservative activist Charlie Kirk as a “martyr” on Sunday in remarks at his memorial in Arizona, but he pivoted swiftly to blunt politics by saying that he hated his political opponents and that they “cheated like dogs.”

Striking a far different tone from that of Mr. Kirk’s widow, Erika, who spoke immediately before him, Mr. Trump said he disagreed with Mr. Kirk’s view of wanting the best for one’s opponent.

“I hate my opponent and I don’t want the best for them,” he said.

He added: “I am sorry, Erika.”

Just minutes before, Ms. Kirk publicly forgave the man who killed her husband. “I forgive him because it is what Christ did,” she said. “The answer to hate is not hate.”

Mr. Trump’s speech, which stretched on for roughly 45 minutes, capped a more than five-hour memorial service for the conservative activist, who was assassinated less than two weeks ago while speaking on a college campus in Utah.

Many members of Mr. Trump’s cabinet and senior White House staff — including Vice President JD Vance, Secretary of State Marco Rubio, Defense Secretary Pete Hegseth and Susie Wiles, the White House chief of staff — also spoke at the event, which drew tens of thousands of people to State Farm Stadium in Glendale, Ariz.

The president, in his remarks, oscillated between honoring Mr. Kirk’s life and promoting his political agenda. Mr. Trump teased an announcement on Monday about autism, discussed sending in federal agents to Chicago to reduce crime and boasted about the tariffs he has slapped on foreign countries.

But Mr. Trump also shared memories of his interactions with Mr. Kirk, telling the story of Mr. Kirk’s rise from a high school graduate with $1,800 in his pocket to building a multimillion-dollar political juggernaut with chapters around the country. That work, Mr. Trump said, was critical to Republicans making inroads with younger voters in the last presidential election.

The president also credited Mr. Kirk for being among the first to tell him about Mr. Vance, when he was running for Senate in Ohio, and for uniting Mr. Trump and Robert F. Kennedy Jr., now the health secretary, at an event in Arizona last year.

Mr. Trump also recognized Mr. Kirk’s role as a father, son and friend — and his dedication to his faith and his work.

“He’d make me feel very guilty,” he said. “As president, many people ask me for things, but Charlie was one of the few who always gave more than he took.”

The president, who had already announced that he would posthumously award Mr. Kirk the Presidential Medal of Freedom, promised to Ms. Kirk that he’d do “everything we can to ensure that your children grow up in a land where their father is honored and revered as a great American hero.”

And though Ms. Kirk said she forgave her husband’s killer, Mr. Trump called him “a radicalized, cold blooded monster.”

The president also took aim at Mr. Kirk’s critics and once again blamed liberal politics for his death. Mr. Kirk spent much of his time engaging with those who disagreed with him, as he sought to win them over. But Mr. Trump assailed those critics, saying that most of them were “paid agitators,” and he vowed that the Department of Justice was “investigating networks of radical-left maniacs who fund, organize, fuel and perpetrate political violence.”

“But law enforcement can only be the beginning of our response to Charlie’s murder,” he said.

Since Mr. Kirk’s death, Mr. Trump and top administration officials have promised a broad crackdown on their political opponents, leveling a baseless argument that there is a broad ecosystem of Democratic individuals and groups violently opposing the Republican Party and its values.

As Mr. Trump’s speech veered increasingly into politics on Sunday, hundreds of people started leaving the arena. Many of the attendees woke up at 3 a.m. and stood in line for hours to make it into the venue, with organizers setting up multiple overflow areas.

Most of the speakers before the president talked extensively of Mr. Kirk’s faith, and they vowed to carry on his mission to implement a conservative Christian vision of the United States.

“We always did need less government,” Mr. Hegseth said. “But what Charlie understood and infused into his movement is, we also needed a lot more God.”

Top administration officials also credited Mr. Kirk and his organization, Turning Point USA, for helping Republicans win elections, saying their work was invaluable to their success.

“Our whole administration is here, but not just because we love Charlie as a friend, even though we did, but because we know we wouldn’t be here without him,” Mr. Vance said. “Charlie built an organization that reshaped the balance of our politics.”

At the end of his remarks, Mr. Trump welcomed Ms. Kirk back to the stage. The two hugged and stood together as “America the Beautiful” played. They then clasped hands and walked off together.