IUKA, Miss. — When Chris McDaniel first ran for Senate four years ago, his campaign became a cause for disaffected and restless Republicans across the country. Activists waving “Don’t Tread on Me” flags flooded Mississippi. Tea Party-aligned groups spent millions supporting him. Donald J. Trump — who was still a year away from announcing his presidential campaign — took notice and tweeted his endorsement: “He is strong, he is smart & he wants things to change in Washington.”

Mr. McDaniel, a state senator and a lawyer, received more votes than any other candidate in that Republican primary, but eventually lost in a runoff to the incumbent, Thad Cochran. But now as Mr. McDaniel embarks on another run for Senate, his campaign contributions are a fraction of what they were in 2014. On a good night, a few dozen people show up to hear him speak. And President Trump is so far keeping his distance from the race.

Mr. McDaniel’s faded political fortunes point up one of the more unforeseen effects of Mr. Trump’s leadership of the Republican Party. Instead of elevating the renegade, insurgent conservatives who have vowed to challenge party leaders in Washington — candidates who are politically and temperamentally cut from the same cloth as the president — Mr. Trump has effectively shut off the oxygen to the noisiest and most fractious wing of his party.

He has endorsed almost every incumbent Republican senator, making it much more difficult for challengers like Mr. McDaniel to wage the kind of primary fights that have sown division inside the party for most of the last decade. In Alabama, Nevada and West Virginia, Mr. Trump has actively worked against candidates that had strong support from grass-roots conservatives.