The two politicians -- top contenders in the battle to become their party's presidential candidate -- offer dramatically different temperaments, campaign styles, and even in their life narratives.
Gingrich, 68, was born to a teenage mother who divorced his father while he was still a baby and married a US serviceman. Gingrich's itinerant childhood on various US military bases included overseas stints in France and Germany.
By contrast, Romney, 64, a monied scion of privilege, was the son of millionaire Mormon businessman George Romney.
The elder Romney became governor of Michigan, US housing secretary of state and chairman of the American Motors Corporation.
George Romney tried and failed in his bid to become US president, becoming a top contender for the 1968 Republican nomination before losing out to Richard Nixon. Now Mitt, who was elbowed out for the 2008 nomination by Senator John McCain, gets his shot at the White House.
Gingrich, a former speaker of the House of Representatives who has surged to the front of statewide polls one week before the January 31 primary here, has a pugnacious style, and is known to verbally pummel opponents into submission with a razor sharp intellect.
As an academic and college professor, Gingrich had a proclivity toward bold political schemes and "big ideas" that led him to engineer his audacious plan for the Republican takeover of Congress in 1994.
But in one of the frequent knocks against him, Gingrich is also criticized as grandiose and overbearing, given to bombast and bluster.
Impulsive even as an adolescent, Gingrich decided at the age of 15 that he would study history and politics after a visit to Verdun, France.
He followed that chosen path, trying and failing twice in his bid to be elected to the House, before succeeding on the third try.
Gingrich represented his Georgia district for several years before being elected speaker by his fellow lawmakers -- a post he held from January 1995 to January 1999, when he was forced out.
His fall, facing a rebellion after a midterm poll rout and ethics violations, was as spectacular as his rise had been impressive.
Even after his ouster, Gingrich never really left the world of politics, creating and chairing several policy think-tanks, and taking on work as a highly paid consultant for the mortgage giant Freddie Mac, which is seen as complicit in the US financial meltdown of 2008.
Where Gingrich runs hot, Romney blows cold. He is steady, patrician, even aristocratic. But he is often parodied as unexciting and overly studied, every bit the trained lawyer and businessman.
With his silver spoon and Ivy League education, Romney amassed a fortune at the head of Bain Capital during the 1980s and 1990s, and now is said to be worth about $250 million.
That would make him one of the richest men ever in the White House -- if he gets the nomination and defeats President Barack Obama in November's election.
Romney failed at his first attempt to gain elective office, when he tried to unseat liberal icon Ted Kennedy from the US Senate.
After being elected governor of Massachusetts for a term, he tasted election defeat again, when he lost out to McCain for the 2008 Republican presidential nomination.
He now faces an unlikely challenge from Gingrich deemed by some too intemperate and controversial to get the nomination.
But Romney -- from all appearances the very picture of everything a candidate should be, down to his graying temples and ram-rod straight bearing -- is seen by many as being too stiff, too perfect.
Taking those criticisms on board, in a departure from four years ago, the candidate has taken to wearing his hear a bit more mussed and his shirts a bit rumpled, to give himself more of the air of an average guy.
There are also lingering doubts in conservative circles that the former governor of liberal Massachusetts really shares their views on hot-button issues such as gay marriage and abortion.
Another Romney vulnerability is his Mormon religion, his bedrock faith, but one which is seen by many Americans as strange and cult-like.
Gingrich, a convert to his wife's Catholic faith, is the more conservative of the two, harping frequently about his adherence to the principles of Ronald Reagan, the late president.
Romney is criticized as lacking core principles, and has shown a willingness to adapt his views those of the electorate when it is convenient to do so.
There is no greater contrast between the two men than in how they have conducted their personal lives.
Romney, an irreproachable family man who has been married for 42 years to his high school sweetheart, has five grown sons and 16 grandchildren.
By contrast, Gingrich is married to his third wife, Callista, a former congressional aide more than two decades his junior, who was his paramour while he was still married to wife number two.
