Editor's note: Ari Fleischer, a CNN contributor, was White House press secretary in the George W. Bush administration from 2001 to 2003 and is the president of Ari Fleischer Sports Communications Inc.
(CNN) -- There are now just two people standing between Mitt Romney and the GOP presidential nomination -- Newt Gingrich and Sheldon Adelson, the billionaire financier of Newt's super PAC.
All the other Republican candidates have faded, like Jon Huntsman, or they're about to fade, like Rick Santorum whose hot Iowa hand didn't last, and Texas Gov. Rick Perry, whose decision to stay in the race is a South Carolina dream come true for Romney, and a real setback for southern politician Gingrich.
Gingrich, despite his many faults, is the only contender with enough stature to make this an ongoing race, especially with his friend's money keeping his hopes alive.
But for Newt to have any chance of stopping Romney, two things have to happen. First, everyone else needs to drop out (with the exception of Ron Paul, whose base is so independent that his appeal to voters in the remaining primaries, many of which have limited or no participation by independents, is diminishing before our eyes.) Second, Newt has to make the tough, personal decision that he's in it for the long haul.
Romney has run a strong race, but there remain many "not-Romney" voters in the GOP whose votes have been fruitlessly split among four or five other candidates. So long as Perry, Santorum and Gingrich are in the game, the game is over and Romney wins.
But if it gets down to a 1:1 proposition, Romney will be tested again. Even if Romney wins South Carolina, despite his money and momentum, he'll still be vulnerable if a 1:1 race were to break out.
Republicans like Romney. They think he's qualified. But they don't love Romney and many worry about his core convictions.
That's why this race will come down to Newt's personal decision. Will he yield after South Carolina, recognizing the GOP needs to unite, or will he keep going, out of sheer determination and knowing Romney's weaknesses?
If he keeps going, it's going to be a long slog, with many Republicans viewing him as a spoiler intent on damaging the party's likely nominee. Many other Republicans though, will look forward to casting an anti-Romney vote that can make a difference. Most won't be voting for Newt, they'll be voting against Romney.
Newt has been around the block long enough to know this. He has little chance to win the nomination, but he has none if he doesn't stay in the race.
The opinions expressed in this commentary are solely those of Ari Fleischer.