For more than three decades, Senator John McCain, who was brutally tortured as a prisoner of war in Vietnam, has been a powerful and persuasive voice in Congress on matters of interrogation and the conduct of war.

But in the twilight of his career, as he battles brain cancer at his Arizona ranch, far from President Trump’s Washington, the potency of his moral suasion has faded as voices of ridicule in his own party rise.

Less than 24 hours after Mr. McCain, Republican of Arizona, released an agonized statement in which he said he could not support the confirmation of Gina Haspel to direct the C.I.A., his best friend in the Senate, Lindsey Graham, Republican of South Carolina, announced his support for Ms. Haspel, who once oversaw a C.I.A. secret prison where a terrorism suspect was tortured.

Senator Susan Collins, Republican of Maine, who is also close to Mr. McCain and previously has sided with him on issues of torture, is also voting for Ms. Haspel.