Swift, 36, decided to leak her decision through aides to the media rather than make a formal announcement in deference to the nation's terrorism crisis, the Boston Globe and Herald said Thursday.
Swift is a member of the Republican Party, which has controlled the governor's office in the overwhelmingly Democratic state for a dozen years. At least five Democrats are expected to vie in a primary next September for the chance to challenge Swift.
Her decision to seek to become the first woman elected governor in the state had been expected to be made public early in September, but that was delayed by the terrorist attacks on New York and Washington on Sept. 11.
Since those attacks, Swift's political popularity has soared due to her strong steps to beef up security at Boston's Logan International Airport and the Port of Boston. A recent Boston Herald poll showed her approval rating, perilously low a year ago when she was the state's lieutenant governor, rose to 66 percent.
Aides said she felt now was the time to disclose her decision to run.
"She made her decision out of a sense of obligation to the party, her supporters and other potential candidates," one aide said. The adviser said that with the election a little less than a year away, "it's important that the party know what her plans are."
Swift automatically became governor in April when Gov. Paul Cellucci became ambassador to Canada and she was seven months pregnant.
Swift gained national attention in mid-May when she gave birth to twins. She and her husband, Chuck Hunt, also have a toddler at home in Williamstown.
Aides said that after she returned from a three-month maternity leave, Swift became convinced she could handle running the state while undertaking a campaign and filling her duties as a parent.
"Shortly after the birth of the children, she wanted to see how well she could juggle the schedule, and she's handling her time well," an aide told the Boston Herald. "It's working for her and her family."