Swift, citing family concerns and political realities, stunned supporters and opponents by telling a noon news conference that she had decided not to run.
"Lest there be any doubt, I'm in," Romney told a news conference outside his Belmont, Mass., home. "The bumper stickers have been printed, the Web site is going up tomorrow (Wednesday) morning, the campaign papers are filed today."
Romney praised Swift for her decision and said he planned to meet with her soon to discuss the issues facing the state.
"In my view Gov. Swift deserves the thanks and praise of the people of Massachusetts," Romney said, adding that he choose to make a low-key announcement out of respect for Swift.
"I want this to be Gov. Swift's day," he said.
He said Swift informed him of her decision by telephone earlier Tuesday, and offered him her support.
"I very much appreciate her support in this race," said Romney, who vowed to bring "the very best possible management" to the state.
"I also think it's admirable that she's decided to focus her resources on managing the state during tough economic times and also helping raise her family with all of her energy and heart," he said.
Earlier Tuesday Swift, sometimes fighting back tears, said no one pressured her to get out of the race, and that her decision was made over the previous 24 hours after discussing the situation with her husband and political advisers. Any pressure "came from inside," she said.
"I've never walked away from a fight in my life," said Swift, but political realities convinced her not to mount a challenge to a well-heeled opponent in what would have been a very aggressive and expensive primary campaign.
With contributions to her campaign dwindling over recent months and polls showing her trailing Romney by more than 60 points, Swift said it was obvious that running against the man who successfully cleaned up the scandal-plagued Winter Olympics Organizing Committee in Salt Lake City was going to be an uphill battle.
She said a primary campaign would have greatly increased the demands on her, taking time away from her husband and their children.
Because "time with family was non negotiable, something had to give," the state's first woman governor said.
Swift said Romney was "very gracious" in their conversation, and that the two will sit down soon to discuss the upcoming campaign to keep the governor's seat in the hands of the Republican Party.
Five Democrats are running for their party's nomination, including Clinton Cabinet member Robert Reich.
"This was a tough decision," said Swift, who inherited the office when Gov. Paul Cellucci became ambassador to Canada last year.
Swift said she plans to be back at the Statehouse on Wednesday "with my armor back on" ready to fight for her priorities of providing the people of Massachusetts with opportunities to improve their lives.
Swift's decision caught many off guard because until now she had promised a strong challenge for the nomination.
The Boston Herald had even reported Tuesday she planned to challenge Romney to a debate prior to the April 6 Republican state convention.
While a recent Herald poll showed Romney leading Swift by more than 60 points among likely GOP primary voters, Swift aides believed she could close the gap by detailing their differences on issues such as economic recovery, abortion, tax rollbacks and budget cuts.
"Where there are differences, we should debate them, we should talk about them," Swift said on Monday.
Swift's flagging popularity received a boost Monday with the endorsement of Republican former Gov. William Weld.
"Her intellect and integrity have impressed me time and time again," Weld said in a statement to the Boston Globe.
"Gov. Swift has my unequivocal support and admiration," Weld said. "I stand foursquare behind her bid for election."
Some political observers believe Romney's popularity resulting from his successful three years as chairman of the Olympic Organizing Committee in Salt Lake City is artificially high and will be vulnerable once he takes positions on controversial issues, including bilingual education and gay marriage.
One of the questions expected to dog Romney was abortion. When he lost in a close race to Democratic Sen. Edward Kennedy in 1994, Romney said during the campaign he supported legalized abortion, a position seen as politically necessary in a state where 80 percent of the voters support abortion rights.
However, while in Utah, Romney said in a letter to the Salt Lake City Tribune that he did not want to be labeled an abortion rights advocate, and had "never been comfortable with the labels associated with the abortion issue."
At Tuesday's news conference, however, he insisted his position has been well known since 1994.
"On a personal basis, I don't favor abortion, but as governor, I will protect a woman's right to choose," he said.
He acknowledged that during the months leading up to the fall general election, there will be people who would bring the campaign down with "ugly issues," but Romney said he intends to keep it at a "high level."
Senate President Tom Birmingham, a Democratic gubernatorial hopeful, said Swift's decision was a surprise. He added that if Tuesday's events are seen as "basically a coronation" of Romney, it could backfire on the Republicans.
"They've (Republicans) been in control (of the governor's office) for 12 years," Birmingham said, "and enough is enough."