He has worked quietly with the White House to apply a conservative policy structure to the unorthodox brand of Republicanism that elevated Mr. Trump to power, urging fellow senators to be mindful of the anti-free-trade, anti-open-immigration energy that will color the lawmakers’ own elections.

He has been a critical back-channel resource on personnel decisions for the administration, recommending Lt. Gen. H. R. McMaster as national security adviser.
He has publicly defended Mr. Trump against all manner of misadventures, earning cascading boos at a town hall meeting back home after parroting the president’s rationale for not releasing his tax returns. (“As far as I’m aware, the president says he’s still under audit,” Mr. Cotton offered, a bit meekly.)

But of late, even Mr. Cotton has struggled to defend all of Mr. Trump’s conduct. And he has been compelled to remind the president of the agenda he won on.