It was the kind of moment that used to make Republicans think: Senator Marco Rubio will not have to suffer this job for long.
He glared at Rex W. Tillerson, the nominee for secretary of state, from behind his committee nameplate, his boyish face just a pinch more weathered than it used to be. He checked off the grievous offenses of President Vladimir V. Putin of Russia with prosecutorial zeal, pressing Mr. Tillerson to brand Mr. Putin a war criminal and acknowledge the mysterious deaths of his political opponents.
When Mr. Tillerson equivocated, saying he hoped to learn more once he could receive classified information, Mr. Rubio’s retort was the stuff of a thousand cable news replays.
“None of this is classified, Mr. Tillerson,” the senator said flatly. “These people are dead.”
With that exchange and two others later in Mr. Tillerson’s rocky nine-hour confirmation hearing last week, Mr. Rubio, 45, reintroduced himself to the legislative body he had hoped, not so long ago, to leave behind for good.
He has emerged as a potentially decisive vote on perhaps President-elect Donald J. Trump’s most consequential nominee, teeing up an early, public test of how adversarial he is willing to be toward a fellow Republican in the White House — albeit one who long demeaned him as “Little Marco.”
For Mr. Rubio, a preternaturally gifted politician chastened by a failed White House run, the choice mirrors the broader question facing Republicans on Capitol Hill: Is the price of defying Mr. Trump — who can sink fortunes 140 Twitter characters at a time — worth paying to serve as a critical check on his expansive powers?
“On most major issues you are never going to make everyone happy,” Mr. Rubio said in an email Monday evening, when asked how the experience of running for president had affected his view of this job. “So you might as well do what you truly feel is right and let the political chips fall where they may.”
Dissent does offer its share of advantages. Mr. Rubio has edged back into the political foreground for the first time since ending his own presidential bid, then reversing a pledge not to run for re-election to his Florida seat. (“I have only said like 10000 times I will be a private citizen in January,” Mr. Rubio wrote on Twitter in mid-May, about five weeks before announcing his change of heart.)
As has happened occasionally in his years on the national stage, Mr. Rubio has appeared willing to rankle elements of his own party for at least a moment, endearing himself briefly to left-leaning skeptics who have generally found themselves disappointed.
“Florida Man Actually Does His Job,” read a Twitter post from the popular @_FloridaMan account, which tracks the misadventures of Sunshine State denizens.
“Good for Little Marco,” Joy Behar, a liberal co-host of “The View,” told viewers last week, after Mr. Rubio’s aggressive questioning spawned a rare morning-show meditation on the Senate Foreign Relations Committee.
By the end of the segment, Ms. Behar appended a warning. “Watch Marco Rubio vote for Tillerson,” she said. “Just watch.”
Among some members of both parties on Capitol Hill, where Mr. Rubio’s convictions on party-bucking issues have at times been called into question, this is still viewed as a possibility.
Memories of Mr. Rubio’s ill-fated push for immigration reform in 2013 remain fresh. So, too, do his halting attempts to distance himself from the legislation during last year’s Republican primaries.
Then there is Mr. Trump, whom Mr. Rubio once called a dangerous “con man” with “the worst spray tan in America,” going so far as to sell “#NeverTrump” bumper stickers on his campaign website.
Weeks later, Mr. Rubio said he would support Mr. Trump if he won the nomination. At that point, he planned to set off on a lucrative private sector career.
“He made a decision that probably cost him a seven-figure income,” Senator Tim Scott, Republican of South Carolina, said of Mr. Rubio’s re-election. “He’s come back with a freshness and an enthusiasm and a real strong desire to make a difference.”
Mr. Rubio and his allies have said that the decision to return stemmed in no small part from a desire to act as a foreign policy check on either Mr. Trump or Hillary Clinton.
At the same time, Mr. Rubio said via email, he is excited to work with a Republican administration “rather than being on defense spending most of our time fighting to prevent bad policy” under President Obama.
Aides noted that Mr. Rubio had come out in favor of several other Trump nominees, playing down any lingering tensions from the campaign.
But as Mr. Tillerson’s confirmation hangs in the balance, Mr. Rubio is still the likeliest Republican defection on the committee, which includes 11 Republicans and 10 Democrats, potentially imperiling Mr. Tillerson’s chances.
It is also possible for Mr. Tillerson’s nomination to be considered by the full Senate without an affirmative recommendation from the committee.
At least a few colleagues are watching closely.
“I certainly weigh heavily Rubio’s perspective on foreign policy,” Mr. Scott said, when asked if Mr. Rubio’s questioning had made him think differently about Mr. Tillerson.
Mr. Scott said he reached out to Mr. Rubio shortly after the hearing to discuss what happened. Asked in an interview which section he hoped to learn more about, Mr. Scott stared back for a beat.
“Did you watch it?” he said, aboard a subway in the basement of the Capitol last week. “O.K. … ”
Mr. Rubio, who was criticized for appearing overly scripted at times during his presidential campaign, is one of a handful of former Republican candidates working to find his way in the Trump era.
Senator Ted Cruz, Republican of Texas, once Mr. Trump’s most blistering rival, has more recently positioned himself as an eager cheerleader, lauding the president-elect’s nominations with characteristic showmanship.
Another former candidate, Senator Rand Paul of Kentucky, has distinguished himself as the lone Republican “no” vote in the early stages of efforts to repeal the Affordable Care Act, opposing a budget measure last week because of concerns over increases in federal spending and debt.
But Mr. Rubio has earned the brightest spotlight. When Mr. Trump chose Mr. Tillerson, Mr. Rubio expressed immediate reservations, citing the nominee’s close ties to Russia while at Exxon Mobil. Aides said he read every speech Mr. Tillerson had given over the past decade in preparation for the hearing.
In a week when some Democrats’ hopes of embarrassing Mr. Trump’s prospective cabinet mostly failed to materialize, several conceded it was Mr. Rubio who drew the most blood.
The damage was not lost on Republicans. Long before the hearing, Tillerson supporters had moved to persuade Mr. Rubio, including through a conversation with former Vice President Dick Cheney.
At the session itself, Senator Bob Corker, Republican of Tennessee and the committee’s chairman, appeared at times to coax Mr. Tillerson toward more palatable answers, plainly eager to shepherd him out of the committee.
As the hearing wrapped up, Mr. Corker joked that “almost every senator” had used the time well, a comment he later said was not directed at anyone in particular.
He allowed, in a brief interview later in the week, that Mr. Rubio’s portion was “a fairly intense questioning period.”
Nearby, a swarm of reporters chased Mr. Rubio to an elevator, lobbing questions about Mr. Tillerson. He deflected them politely, saying he would not be “commenting on the process,” but lingered as the crowd grew.
At last the doors opened. The senator stepped on, smiling at least a little.