President Trump sought to dismiss concerns about rising gas prices from his war in Iran, contending that the price spike “doesn’t really affect us” even as some Republicans fear that soaring costs are undercutting their economic argument heading into the November elections.

Mr. Trump, speaking in Florida on Monday, said that prices were “artificially up” because of the war and vowed that they would drop again after it ended. But the president offered no clear timeline of when that would happen, even as he bragged that the United States was far ahead of its timeline.

Mr. Trump’s economic message on Iran so far boils down to the idea that any pain is short-term, worthwhile and not as bad as he initially thought it would be. But for Republicans in tight midterm races, the knock-on effects of the war — and Mr. Trump’s so-far vague assurances about the economy — are still open questions.

The conflict has sent the price of gasoline surging, and it reached $3.48 a gallon on Monday, nearly a 17 percent increase since the war began on Feb. 28. World leaders are preparing for far-reaching repercussions, and in private some Republicans lawmakers are growing resigned to the harsh political realities that confront the party in power when prices rise sharply.

Senator John Thune of South Dakota, the Republican majority leader, told reporters on Monday that “the price of gas is always kind of a benchmark.”

“I do think the fact that we’ve increased our supply here domestically will help ease it, but it’s something obviously we’ve got to pay attention to,” he said.