For Mr. Corker, whose surest path to re-election would most likely have involved hugging the president tight, the decision not to run has removed any electoral constraint against full candor. The choice also spares him the potential threat of a difficult primary challenge from his right.

All year, Mr. Corker has strained to maintain a careful balance. As chairman of the Foreign Relations Committee, he has forged close relationships with administration officials like Rex W. Tillerson, the secretary of state. And since last year, he and Mr. Trump had appeared to achieve something of a mutual respect themselves — the president finding in Mr. Corker another fortune-making builder and self-styled dealmaker in construction and real estate, who thrilled as much at the chase as at the profit.

“Do you think if we gave it back to them,” Mr. Corker once asked an associate, Michael Compton, moments after buying two of the largest real estate companies in town, “they’d let us try to buy it again?”

Before their latest flare-up, Mr. Corker and Mr. Trump were on semiregular speaking terms, golfing together this year with the retired N.F.L. quarterback Peyton Manning. “He is not like people think that he is,” Mr. Corker said of Mr. Trump at one point during the campaign, praising him as “courteous, kind, respectful.”

Yet after wrestling with how thoroughly to lash a president whose temperament he had come to doubt (he did, during the spring, suggest that the White House was in a “downward spiral”), Mr. Corker has turned to total bluntness, the logical extension of a professional life spent in near-permanent states of restlessness and cost-benefit analysis.

The evening before he said he would not seek re-election, Mr. Corker held a fund-raiser near the Capitol. One by one, guests raised questions about the Republican agenda, and what could reasonably be accomplished in Congress. Repeal-and-replace? Entitlement reform?