In a letter, John Curtis of Utah called for the Senate Judiciary Committee to subpoena the sons of President Trump and former President Joseph R. Biden Jr. Senator John Curtis called for the Senate to investigate whether Donald Trump Jr. exploited his access to his father for personal gain, spurred by reports that a Russian oligarch funded a lavish wedding after-party.

In a three-page letter, Senator John Curtis, Republican of Utah, urged the Judiciary Committee to subpoena Donald Trump Jr., after it was revealed that a Russian businessman with ties to the Kremlin paid hundreds of thousands of dollars for the festivities on an island in the Bahamas this year. Issuing such a subpoena would be exceedingly unlikely by the Republican-led panel, and top Republicans would not say on Tuesday whether they planned to take action.

But the request by Mr. Curtis marked a rare instance of a G.O.P. lawmaker daring to challenge the president. The president has said that his son would repay the Russian businessman, Umar Kremlev, for what he called a wedding gift. But Mr. Curtis rejected that characterization.

“A toaster is a wedding gift,” the senator said in a social media post. “A private-island party paid for by a Putin-connected oligarch is something else. Republicans nearly wore out the subpoena machine investigating Hunter Biden’s foreign relationships. We should not unplug it now.”