As Senate Republicans begin to grapple with the unappealing prospect of serving as jurors in the impeachment trial of a president of their own party, Mr. Romney has emerged as a crucial figure.

“He’s in a different place from many politicians still feeling out their place in the party and hoping to be president someday,” said Senator Christopher S. Murphy of Connecticut, a Democrat who has become friendly with Mr. Romney from their service together on the Foreign Relations Committee. “He’s a loyal Republican, but that’s not his first priority — he’s a bit of a throwback.”

For Mr. Romney and Mr. Trump, it is the latest, and perhaps last, installment of an off-and-never-really-on relationship between two men who shared little more than the same political ambition. It began when Mr. Romney, as the 2012 Republican presidential nominee, solicited Mr. Trump’s endorsement, and carried through 2016, when the former Massachusetts governor began the year as a ferocious critic of Mr. Trump but ended it as a supplicant, sharing dinner with a president-elect in hopes of becoming his secretary of state.

Now, though, Mr. Romney holds a crucial position of power. He could be part of an effort to eventually remove Mr. Trump from office, and is already in the ear — and perhaps conscience — of other Republicans.

He plays down his role, noting that he is not trying to lead any sort of insurrection against the president.

“I spoke out because I believe this is a matter of importance and personal principle,” he said. “Nothing more, nothing less.”

But by reproaching Mr. Trump, he offers Democrats the ability to counter claims that the inquiry is a wholly partisan and politically fueled witch hunt, as the president has repeatedly called it. And Mr. Romney both provides cover for and exerts pressure on his fellow Republicans, who are anxiously calibrating what to say about a scandal that only deepened on Thursday when audio emerged of Mr. Trump privately suggesting that government officials who expressed concerns about his dealings with Ukraine deserve to be punished severely.