Mitt Romney's decisive victory in Nevada was never much in doubt. He won the state's caucuses four years ago, kept his organization active and he could count on support from Mormons who made up a quarter of caucus goers this year.

But the former Massachusetts governor's win here, coupled with his enormous Florida victory just days ago, proved Republicans have begun to coalesce around his candidacy in earnest. He swept nearly every voting group in Nevada including those that have been slow to come aboard, such as tea party activists and voters who describe themselves as extremely conservative.

And that spells trouble for his remaining rivals who now face a stark question: How do they stop him?

It's becoming increasingly difficult for Newt Gingrich, Ron Paul and Rick Santorum to break through. All trail Romney in money, momentum and organization in upcoming states. And with a calendar of primaries and caucuses favorable to Romney just ahead, no one else has a clear path to the nomination.

"February is really the death march for second and third tier candidates," Republican strategist Rich Galen said. "If Romney sweeps February, the arc of his effort will be so strong, for most Republicans it will be over."

A handful of states hold caucuses next week, including Maine, Minnesota and Colorado. Then comes a three week lull until primaries in Feb. 28 in Arizona and Michigan, where Romney grew up and where his father served as governor.

Romney's confidence was clear as he stepped before cameras to claim his Nevada win, focusing on President Barack Obama.

"I've walked in Nevada neighborhoods, blighted by abandoned homes, where people wonder why Barack Obama failed them. Mr. President, Nevada has had enough of your kind of help," Romney said to cheers.

He barely acknowledged his GOP rivals, Gingrich included.

The former House speaker's hope for a serious head-to-head contest with Romney diminishes with each loss.

Gingrich is short on cash, and Southern states likely to be most receptive to his candidacy do not hold contests again until Super Tuesday, March 6. Gingrich's one victory so far came in South Carolina's primary January 21.

Restore Our Future, the deep-pocketed super PAC backing Romney, continues to pummel Gingrich with television ads attacking his record in Washington. The group has purchased advertising time in Minnesota, Michigan and Arizona.

The candidates will debate just once this month, in Mesa, Ariz., on Feb. 22. That hurts Gingrich, who has seen his political fortunes rise and fall on the strength of his debate performances.

Nevertheless, Gingrich brushed aside all talk of quitting the race, saying he was working to "find a series of victories which by the end of Texas primary will leave us at parity" with Romney by early April. He said he and his aides have spent much of the past four days retooling a campaign that twice has made him a leader in the polls, yet left him with only one victory in five states.

Gingrich may be able to hang on if casino mogul Sheldon Adelson makes another large contribution to Winning Our Future, a super PAC supporting Gingrich. Adelson and his wife have contributed $10 million to the group so far. But even that won't make Gingrich's path to the nomination any more plausible.

Paul, who campaigned hard in Nevada and has staked his entire strategy on winning caucus states, may be bordering on irrelevancy as a serious contender after this shellacking. By raising expectations for a strong finish in Nevada — only to place a weak third — the Texas congressman undercut himself going into the next round of caucus states.

Paul's ultimate goal — keeping his libertarian-leaning platform part of the national political conversation — could also be damaged as his losses pile up.

Santorum has watched his candidacy lose steam since his win in the Iowa caucuses early last month.

The former Pennsylvania senator spent little time campaigning in Nevada; his strong social conservatism was never a good fit in a state best known for drinking, gambling and legalized prostitution. He's looked ahead to caucuses in Colorado and the non-binding Missouri primary next week instead.

Santorum continues to campaign on the cheap, and is pulling in enough money — $1 million alone last week, his advisers say — to keep his shoestring operation alive. But few states outside of Iowa have proved to be receptive to Santorum's sharp focus on social issues like abortion.

To be sure, Romney continues to face challenges of his own.

He was at his best as a candidate after Gingrich won South Carolina and appeared to be a real threat going into Florida and beyond. Romney quickly retooled his approach, sharpening his attacks on Gingrich on the campaign trail and in two back-to-back debates.

But without a strong rival, Romney's weaknesses as a candidate and his propensity for gaffes become magnified.

With the recession-weary public still angry at Wall Street and big business, Romney has struggled to explain how the millions he made at the venture capital firm Bain Capital make him a plausible job creator.

He made an eye-popping unforced error this week, telling CNN "I'm not concerned about the very poor." He went on to explain himself, saying those most in need can rely on social programs and that he wants to be a champion for the middle class instead.

But the comment, coming from a man of such immense wealth, struck a sour note — particularly as he campaigned into Nevada, which has the highest unemployment and home foreclosure rate in the country.

Romney attempted to clarify his remark later, saying he "misspoke." But Gingrich has already used it as an attack line, as have Democrats working to weaken Romney.

Still, Romney's position coming out of Nevada is strong and getting stronger.

"When you match this up with Florida, it may well mean this is the beginning of the triumphant march to the nomination," said Galen, the GOP strategist.