The rally, which had been rescheduled after the deadly Parkland, Fla., shooting last month, capped a frenzied stretch of fund-raising and campaigning on behalf of Mr. Saccone, who is locked in a tight race with the Democratic nominee, Conor Lamb.

“He’s an extraordinary person,” Mr. Trump said of Mr. Saccone, dismissing Mr. Lamb as “Lamb the Sham.” “The people of Pittsburgh cannot be conned by this guy Lamb, because he’s not going to vote for us.”

It was an opening for Mr. Trump not only to try to apply his political sway in the year’s first special election — one being looked at as a bellwether for the November midterm elections — but also to seize the opportunity to promote his new tariffs before a steel-country base of supporters far more receptive than the officials and pundits in Washington.

“Steel is back,” he proclaimed more than once. “Aluminum is back.”

He also, after receiving some boos upon first mentioning Mr. Kim, the North Korean dictator, said that he thought the North Koreans “want peace.” He said that his unprecedented meeting could go two very different ways: “We may leave fast,” or strike one of history’s greatest deals.

While the rally was not specifically advertised as one for Mr. Saccone, a loss by him would mark the third time that Mr. Trump had thrown his political might behind a candidate without success. With his low poll numbers, Republican leaders and the White House are trying to determine how to handle his desire to campaign in the midterms.

But he was a hit with the crowd on Saturday. He retreated to fan favorites: reminiscing about his Electoral College victory, and saying that there is a “lot of evil in Washington” and that he is “getting it out.” To cheers and laughter, Mr. Trump hurled some old and new slurs and insults toward his opponents as he complained about his media coverage and perceived Democratic obstruction.

He embellished his derisive “Sleepy Eyes Chuck Todd” nickname for the “Meet the Press” host by calling him a “sleeping son of a bitch.” He said “Pocahontas” — his name for Senator Elizabeth Warren, Democrat of Massachusetts — would produce bad television ratings if she challenged him in 2020. And he dismissed Representative Maxine Waters, Democrat of California, as a “low-I.Q. individual,” delivering, in a predominantly white region, an insult that to many carries racial undertones.