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Associated Press
Republican presidential candidate, former Massachusetts Gov. Mitt Romney greets supporters at his Nevada caucus night victory celebration in Las Vegas, Saturday, Feb. 4, 2012. At left is his wife Ann and right is his son Josh, along with several grandchildren. (AP Photo/Gerald Herbert)

LAS VEGAS — Mitt Romney's big win in Nevada four days after his breakthrough victory in Florida spells big trouble for his remaining rivals. Simply put: Is there any way to stop Romney now?

There is no question now that Republicans have begun to coalesce around the former Massachusetts governor's 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.

Ron Paul and Rick Santorum are fading fast and it's becoming increasingly difficult for Newt Gingrich, who had been Romney's top rival, to chart a clear path to the nomination. He trails Romney in money, momentum and organization in upcoming states. And a calendar of primaries and caucuses favorable to Romney lies ahead.

"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 on 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 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.