WASHINGTON — He was once described, for better or worse, as the Republican Barack Obama — a fresh-faced first-term senator who just might walk away with his party’s presidential nomination. Instead, SenatorMarco Rubio of Florida returned to his day job on Thursday.

Two days after a disappointing second-place finish in his home state prompted him to bow out of the presidential race, Mr. Rubio was back at the Capitol, falling once more into the routine of roll call votes and conference lunches.

He questioned State Department officials about arms-control issues during a Senate Foreign Relations Committee hearing. He voted for a resolution holding the website BackPage.com in contempt for being complicit in sex trafficking. He attended a weekly Republican luncheon. And now that his run for president is over, he made it clear that he had no interest in several rumored backup plans.

“I’m not going to be vice president,” he said. “I’m not interested in being governor of Florida. I’m going to finish up my term in the Senate over the next 10 months. We’re going to work really hard here, and we have some things we want to achieve, and then I’ll be a private citizen in January.”

It had been several weeks since Mr. Rubio had been spotted in the halls of Congress. In early February, he and Senator Ted Cruz of Texas, then a rival for the nomination, flew in to vote for a bill imposing strict sanctionsintended to derail North Korea’s nuclear ambitions. It passed, 96 to 0.

Lawmakers held that evening vote open for Mr. Rubio, who rushed in about 45 minutes after voting began. “You made it,” one colleague called out as a group of senators circled him, grinning and patting him on the shoulder. (During the campaign, Mr. Rubio faced criticism for the votes he had missed during his Senate tenure.)

Mr. Rubio is the third Republican senator to return to the Capitol after bowing out of the race, joining Rand Paul of Kentucky and Lindsey Graham of South Carolina. Mr. Graham garnered a few headlines of his own on Thursday with the revelation that he will attend a fund-raiser next week for Mr. Cruz. Just two months ago, Mr. Graham said that choosing between Mr. Cruz and his rival Donald J. Trump was like a choice between being shot or poisoned.

Though Mr. Rubio has largely missed the debate on Capitol Hill over President Obama’s Supreme Court nominee, the senator made it clear Thursday that he was up to speed. Expressing his opposition to the nomination of Judge Merrick B. Garland, he said that while he was “more than happy to talk to anybody,” he — like most of his Republican colleagues — saw no point in meeting with him.

Reporters thronged to him as he headed into a closed-door session of the Senate Intelligence Committee, probing for a post-mortem of his campaign: What had gone wrong? Messaging, he replied — he had offered a vision that did not resonate with Americans who are still suffering from the economic downturn.

“I understood that we could have run a campaign based on speaking to people’s frustrations and anger and getting them angrier or more frustrated,” he said. “But I really continue to believe that these disruptions in our economy, while incredibly painful, also provide opportunities.”

Mr. Rubio declined to endorse any of his rivals, saying only that Mr. Cruz’s positions were “conservative” and that he admired Gov. John Kasich of Ohio.

“I think there’s time to still prevent a Trump nomination, which I think would fracture the party and be damaging to the conservative movement,” he said.

Mr. Rubio received a warm welcome from his staff members, who cheered as he arrived at his office Thursday morning, briefcase in hand. “Get back to work,” he told them with a grin.

After 11 months on the campaign trail, a few days of R&R await him: The Senate adjourned on Thursday for a two-week recess.