Mitt Romney told reporters here Tuesday that his vice presidential search team is vetting Sen. Marco Rubio (Fla.), revealing that one of the Republican Party’s biggest young stars is under consideration to be his running mate.

“Marco Rubio is being thoroughly vetted as part of our process,” Romney said after gathering a press pool for an unannounced campaign stop in Michigan.

Romney’s comments came after ABC News reported that his search team had not asked Rubio to complete a questionnaire or submit any personal financial documents. One outside Romney adviser confirmed that report to The Washington Post later Tuesday, though the adviser left open the possibility that Romney officials could thoroughly vet Rubio at a later date.

Romney strongly denied the ABC report, which was attributed to two anonymous sources.

“I can’t imagine who such people are,” the presidential candidate told reporters. “But I can tell you this: They know nothing about the vice presidential selection or evaluation process. There are only two people in this country who know who are being vetted and who are not, and that’s Beth Myers and myself. And I know Beth well. She doesn’t talk to anybody. The story was entirely false.”

Romney campaign spokesmen said they did not know when or whether Rubio submitted his vetting paperwork, adding that the process is being kept confidential by Myers, a longtime Romney confidant who has been running the search for two months.

Earlier Tuesday, a second adviser who works directly on the campaign told The Post that Romney officials had conducted a preliminary review of Rubio, mostly using documents, statements and news reports that were publicly available. The team did similar public vettings of a large number of other candidates.

Other vice presidential possibilities, including Sen. Rob Portman (Ohio) and former Minnesota governor Tim Pawlenty, are undergoing a more intensive review, according to two Republicans close to the campaign.

Top aides to Rubio declined to comment on the vice presidential candidate search.

Many conservative activists favor Rubio, who often wins vice presidential straw polls.

On Monday, Romney said he would not base his decision on a candidate’s popularity among conservatives but on the person’s readiness for high office. “My criteria is who can be president if that were necessary,” he told the Des Moines Register.

Romney regularly talks about Rubio in his stump speeches, and last Friday he used the senator’s statement on President Obama’s new immigration policy as a starting point for discussing his own views on the subject. Yet throughout the spring, some of Romney’s advisers tried to tamp down speculation that Rubio was a favored pick for vice president.

“By the time you apply the gravitas test, which is really 95 percent of what Governor Romney’s looking at — (vice-presidential) people, when introduced to America, nobody would think twice about their ability to be president if necessary — that wipes out 90 percent of the field,” said one outside adviser close to the Romney operation who spoke on the condition of anonymity to talk candidly about the selection process.

The adviser said other Republicans once presumed to be contenders, including Sen. Kelly Ayotte (N.H.) and New Mexico Gov. Susana Martinez, fall into this category.

Other more-experienced candidates said to be under consideration include Rep. Paul Ryan (Wis.), the House Budget Committee chairman, and Louisiana Gov. Bobby Jindal.

Romney, who was vetted by John McCain in 2008 for the running mate spot but lost out to then-Alaska Gov. Sarah Palin, has told confidants that he does not want to put a candidate through an intensive review unless he is considering him or her for the job.

“It’s a decision that’s going to be made by basically one person, and there’s a lot of data gathering going on to put those choices out in front of that person,” said one senior adviser to Romney who spoke on the condition of anonymity to discuss the process. “It’s going to be orderly, and it won’t happen, I don’t think, before July 4th. But they’re ready.”

Rubio, 41, was elected to the Senate in 2010 after serving as speaker of the Florida House of Representatives. The eloquent Cuban American was an instant star in the tea party movement.

“Blue skies ahead,” the outside Romney adviser said of Rubio. “But could he be president tomorrow? I don’t think so.”

Even as his political mentor, former Florida governor Jeb Bush, said the senator would be his pick for vice president, Rubio insisted that he had no interest in joining the national ticket.

In late March, when he endorsed Romney, Rubio told Fox News: “I don’t believe I’m going to be asked to be the vice presidential nominee. That’s not what I intend to be, that’s not what I want to be, and that’s not what is going to happen.”

Still, if he looks past Rubio, Romney risks angering Latinos and conservative activists who hope to see the young star on the ticket.

Portman, Pawlenty and Ryan have been far more active on the campaign trail for Romney than Rubio has. Pawlenty spent two days traveling with Romney on his campaign bus last weekend, through New Hampshire and Pennsylvania, while Portman and his wife spent Sunday with Romney and his wife, Ann, as they campaigned across Ohio. And Ryan joined Romney on Monday in Wisconsin.

Ryan and Portman are scheduled to host a Romney fundraiser next week in Washington targeting young professionals, while Pawlenty regularly represents the candidate on political talk shows and at finance events.

Their work has been noted by Romney’s advisers; one of them called Pawlenty “an incredible warrior.” The former Minnesota governor, who was a runner-up in McCain’s veepstakes, has emerged as a favorite among some of Romney’s staffers because he is loyal to the candidate, is consistently on-message and has few airs about him. On the campaign bus this weekend, he chatted with staffers about rock music.

But many Republicans close to Romney consider Portman to be the front-runner for the nod. The outside adviser said that as Romney’s team looks deeper into Portman, they find more that appeals to them — including, the adviser added, that Portman speaks fluent Spanish.

“Anybody that scratches the surface a little bit on Portman starts liking him more and more the deeper they go,” the adviser said.

Rubio has far less of a relationship with Romney. They have campaigned together just once, in Pennsylvania in April. The senator has paid more attention to nurturing his own political pursuits. This month, he will be promoting his new memoir, with a book tour planned through several key presidential voting states.

During his five-day bus tour, Romney said he has been hearing a lot of advice from supporters at his rallies about who he should pick.

“In Wisconsin, I get a lot of people saying, ‘Paul Ryan ought to be your VP,’ ” Romney said. “When I’m in Ohio, I get Rob Portman.”