In Texas, some grass-roots conservatives are vocally frustrated with Mr. Trump’s backing of Gov. Greg Abbott, even booing Mr. Abbott when he took the stage. In North Carolina, Mr. Trump’s behind-the-scenes efforts to shrink the Republican field to help his preferred Senate candidate failed last week. And in Tennessee, a recent Trump endorsement set off an unusually public backlash, even among his most loyal allies, both in Congress and in conservative media…

“Things feel like they’ve been shifting,” said Patrick Ruffini, a Republican pollster who regularly surveys Mr. Trump’s standing in the party. “It’s a strong attachment. It’s one that very likely would win a Republican primary today. But is it the same ironclad, monolithic, Soviet-like attachment that we saw when Donald Trump was the incumbent president? No, it is not.”

In a recent Associated Press survey, 44 percent of Republicans said they did not want Mr. Trump to run for president again, while a potential G.O.P. rival in 2024, Gov. Ron DeSantis of Florida, has narrowed the gap in other way-too-early snapshots of a hypothetical primary — new signs of potential vulnerability for the former president.