From western Iowa where he was campaigning, Romney, in turn, praised Haley for her efforts to change government. He said the endorsement was "an honor."
The two planned to campaign together later Friday in South Carolina, a state Gingrich is running aggressively in and that seems to be a more natural fit for the former House speaker and one-time Georgia congressman than for Romney, a former governor of Massachusetts who has struggled to win over South Carolina Republicans.
Haley's nod, somewhat rare because sitting governors of important primary states usually remain neutral, came hours after the candidates met in Iowa for their final debate before the lead-off Iowa caucuses on Jan. 3. The meeting featured Gingrich on the defensive over his decades in Washington, his post-congressional career as a consultant, and whether he was conservative enough to be the Republican Party standard-bearer.
The fast-paced debate underscored the state of the race, with Gingrich atop the polls nationally and in Iowa and Romney, Texas Rep. Ron Paul and Gingrich's other pursuers working in television ads and elsewhere to overtake him.
All were making final pitches to voters on Friday heading into the final weekend before people begin to tune out politics for the holiday season.
Gingrich was returning to Washington, while others looking for late-game surges - like Texas Gov. Rick Perry and Minnesota Rep. Michele Bachmann - embarked on bus tours that would take them across Iowa. They were setting out to make the case that Gingrich is the wrong fit for a party whose core voters are conservative.
Indeed, the big question in the opening moments of a fast-paced, two-hour debate went to the heart of a dilemma that could eventually settle the race: Do conservative Republican caucus and primary voters pick a candidate with their hearts, or do they look elsewhere if they conclude their favored candidate might not be able to defeat Obama?
Those voters begin making that choice in Iowa on Jan. 3, and if experience is any guide, one or more of the presidential hopefuls will not make it out of that state to compete in the New Hampshire primary a week later, let alone South Carolina on Jan. 21.
Haley's endorsement could help Romney in the state that will be the third to weigh in on the GOP field. How much, however, is unclear. Governors can leverage their statewide political networks to help presidential candidates. But polls show Haley's popularity has waned since she won office in November 2010.
Even so, she remains a favorite of tea party activists, whose energy helped Republicans win across the country last fall and whose enthusiasm will be critical in helping the GOP presidential nominee next year. Romney has struggled to court them.
South Carolina is a difficult state for Romney; he competed aggressively there when he first ran for president in 2008 only to bail shortly before the primary when it became clear that he would get crushed after to failing to ease voter concerns about his Mormon faith and his reversals on social issues.
Haley's ties with Romney run deep.
She endorsed him in 2008 when she was in the state legislature. Romney returned the favor in 2010 when she ran for governor in a year when the tea party wielded big clout in key races across the country, hers included.
Haley has said repeatedly that she would choose "the person who should win, not the person that could win. We will endorse the person that will get the country back on track. ... It's all based on policy."