Just a short week ago, Mitt Romney had made history by sweeping the Iowa and New Hampshire presidential nominating contests, and was poised to go three-for-three when he picked up a seemingly inevitable win in South Carolina, says Dan Balz at The Washington Post. Then his Iowa win was revoked because of a vote-counting error, followed shortly by Newt Gingrich's "stunning victory in South Carolina" on Saturday, when he crushed Romney 40 percent to 28 percent. Here, five ways Gingrich's come-from-behind win changed the GOP contest:

1. Romney's GOP sprint is now a long, hard slog
"If Romney had won South Carolina, the race for the Republican presidential nomination would have almost certainly been over," says Chris Cillizza at The Washington Post. Now it's a marathon. After Gingrich's Palmetto State romp, "we are in for — at least — another six weeks of campaigning," through Super Tuesday on March 6. Probably longer, says John Heilemann at New York. Romney has plenty of money, and he won't drop out because "this is his last chance to be president." Gingrich has little cash but a "magnet-like capacity to draw free media," plus an outsized sense of his own destiny. And Gingrich and Romney "are quickly coming to hate each other. So buckle up; this should be fun."

2. Florida is the new tiebreaker
Rick Santorum (belatedly) won Iowa, Romney won New Hampshire, and now Gingrich has taken South Carolina. That means Florida's Jan. 31 primary "will almost certainly decide the nominee," says Hugh Hewitt at National Review. The upcoming Florida "brawl" could well be "the pivotal moment of the campaign," agrees Alexander Burns at Politico. With his money and organizational advantages, Romney starts out as the "muscular favorite" to win the Sunshine State, and he'll need the victory to reassure his panicking backers. But Newt's a good fit for Florida's Tea Party–leaning GOP electorate, and if he can ride his wave of momentum to a win — the latest polls show Newt skyrocketing into the lead — Gingrich's "back-from-the-dead candidacy could become a true juggernaut." 

3. Santorum's prospects look bleak
Newt didn't just crush Romney in South Carolina. By dominating "the other not-Romney candidates, Gingrich took a big step toward consolidating that part of the electorate," says Jennifer Rubin at The Washington Post. Santorum's 17 percent third-place showing is probably enough to keep his campaign alive through Florida, but if he "can't catch either Romney or Gingrich in Florida his campaign becomes problematic." With its large evangelical Christian vote, South Carolina "was Santorum's best chance" to stay in the race, says William Jacobson at Legal Insurrection. If he doesn't drop out, he'll just "play the spoiler, continuing to split the conservative vote." 

4. The GOP elite may panic and court a "white knight"
If Gingrich crushes Romney in Florida, "the Republican Establishment is going to have a meltdown that makes Three Mile Island look like a marshmallow roast," says New York's Heilemann. Rather than risk Gingrich rolling over the more-electable Romney in other states, says Steve Kornacki at Salon, the panicking GOP elite might just try drafting a "white knight" candidate to swoop in and win late big-state primaries to at least stop Newt from winning outright. South Carolina already proved that the GOP base won't accept a "milquetoast moderate from Massachusetts" like Romney, says Erick Erickson at RedState. So if the party leaders won't accept Gingrich, they should force "a brokered convention and find someone acceptable to everyone."

5. The race is about to get really ugly
Despite its reputation for dirty politics, South Carolina "was, by historical standards, decidedly tame" this year, says The Washington Post's Chris Cillizza. But things are "going to get real nasty, real quick" now. Romney has $19 million to spend on making sure Gingrich doesn't win Florida, and his super PAC allies have millions more. If Gingrich can raise money off his South Carolina win, he "will respond in kind" and "fight Romney to the political death in Florida." Bottom line: "If you hate negative campaigning, you may want to turn your television off for the next few weeks. Or maybe months."