Romney scrapped plans for an announcement with supporters at a Boston hotel and instead held a subdued news conference in front of his Belmont home. He thanked Swift for her years of work and said he didn't want to take the attention away from her for this day.
"I also think it's admirable that she's decided to focus her resources on managing the state during tough economic times and also to help raise her family with all her energy and heart," Romney said, surrounded by his wife, Ann, and other family members.
But he was quick to add: "Lest there be any doubt, I'm in. The bumper stickers are printed, the Web site's going up. The papers are going in today."
Swift, who has been plagued by personal and political controversies, said she decided not to take on the simultaneous task of being a mother of three, running for election and governing during a budget crunch.
"Having said early on the time with family was non-negotiable, something had to give," Swift said at news conference earlier Tuesday at the Statehouse. She plans to serve out her term until the end of the year.
"There isn't a working parent in America that hasn't faced it — when the demands of the two tasks that you take on both increase substantially, something has to give," she said.
Recent polls have shown Swift trailing the 55-year-old millionaire venture capitalist by 60 points among likely voters in a Republican primary. The polls also showed Romney beating all the Democratic candidates, albeit by much slimmer margins.
In April, Swift became Massachusetts' first woman chief executive, and, at 36, the nation's youngest, succeeding Paul Cellucci when he became ambassador to Canada.
Until recently, she had said she was undaunted by a challenge from Romney.
Late last month, after a Republican activist had tried unsuccessfully to get her a job in the Bush administration to ease her out of the governor's race, Swift told reporters: "I guess I should be accustomed to powerful men trying to tell me that they know better than I do what it is I should be doing."
Romney, whose father was governor of Michigan, had previously said he wouldn't challenge an incumbent Massachusetts governor, but that changed after Swift's poll numbers plummeted.
The most recent poll showed Romney leading Swift 75 percent to 12 percent. The Boston Herald poll of 401 likely Republican voters was conducted Friday and Saturday and had a margin of error of 5 percentage points.
Swift became the first governor in the nation's history to give birth in office when she had twins in May. She took more than a month of maternity leave and returned to work at the end of June, commuting more than 300 miles roundtrip to her home in Williamstown.
The births produced a wave of favorable national publicity and a bounce in the polls for Swift, who had suffered through a scandal-filled lieutenant governship.
But when she returned to work she continued to find herself with little political capital and a Statehouse dominated by Democrats.
That made tough challenges even tougher: a worsening budget crunch, calls for added security measures after Sept. 11, and a controversial decision not to commute the sentence of convicted child molester Gerald "Tooky" Amirault.
As lieutenant governor, Swift's popularity had plummeted after it was revealed she had asked workers on her staff to baby-sit her daughter Elizabeth. She also used of a State Police helicopter to fly home for Thanksgiving to care for her sick daughter.
The state Ethics Commission ruled she had created an appearance of impropriety by allowing aides to baby-sit her daughter at Swift's home and ordered her to pay a $1,250 fine.
Romney has been riding a wave of popularity since successfully leading the Winter Olympics. A longtime Massachusetts resident who graduated from Brigham Young University in Utah and sent his children there, Romney graduated from Harvard Business School and went to work in Boston at Bain & Co., where he rose to chairman.
In 1984, he co-founded Bain Capital, a venture capital company. He has been active in Belmont's Mormon community, which constructed a 70,000-square-foot temple in 2000.
Five Democrats also are running for governor: Senate President Thomas Birmingham, former U.S. Labor Secretary Robert Reich, state Treasurer Shannon O'Brien, former state Sen. Warren Tolman and Steve Grossman, a former national Democratic Committee chairman.