DES MOINES — Ted Cruz and Donald Trump do agree on one thing—if Trump wins Iowa, he might be unstoppable on his way to winning the Republican presidential nomination.

And they are not alone.

A wide range of senior Republicans told POLITICO that if Trump wins Iowa, he'll more than likely be the nominee. One factor they repeatedly pointed to: An Iowa victory over Cruz would validate opinion polls showing him in command of the race. The Trump phenomenon would officially become a reality.

Mike McSherry, a longtime campaign consultant and former executive director of the Republican Governors' Association, said: "If Trump wins Iowa, I don't know how you'd stop him. All these guys are going to be chewing each other's throats out for second place."

A top official of a rival GOP campaign, speaking anonymously to avoid offending his candidate, said: "If Donald Trump wins Iowa, I think he has won—period. Ted Cruz is supposed to win Iowa. If Trump wins, he'll be on a trajectory to come out of the SEC primaries [March 1] with close to triple the delegates of anyone else."

One of the nation's best-known Republican strategists, speaking on condition of anonymity because of his close connections to the leaders of several of the campaigns, said: "Only Trump can stop himself. All his opponents can do is cause him to do things that make him less appealing."

For weeks, Trump has been telling his crowds here that he thinks a win in the Feb. 1 caucuses would enable him to "run the table." This week, with Trump surging past him in the polls, Cruz, the once-clear Iowa frontrunner warned supporters that if Trump wins in Iowa Monday and eight days later in New Hampshire, where he holds an even larger lead, "there is a very good chance he could be unstoppable and be our nominee."

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These two anti-establishment rivals are both using the same line to motivate their own supporters—obviously, to different ends. But even a growing number of mainstream Republicans, those who have long fretted about the staying power of Trump and the prospect of Cruz emerging as his strongest challenger, have come to believe it.

Asked if Trump can be stopped if he wins Iowa and New Hampshire, Bruce Haynes, a GOP consultant and the CEO of Purple Strategies, was succinct: "Probably not," he said.

A win in Iowa, where just months ago Cruz was sitting on what seemed to be an insurmountable lead, would send Trump into New Hampshire, where he's held a large lead for months, with unmistakable momentum.

"If he was running dead even with Cruz or Rubio or someone in New Hampshire okay, yeah he still has momentum. But he's got momentum on top of these crazy numbers he's held forever in New Hampshire," said Ted Jackson, a GOP strategist in Kentucky. "I mean, that would be a lot to stop. A lot.

"What are you going to do? I mean is there one more commercial, one more endorsement, one more thing that you can say to stop the guy from getting into New Hampshire after he prevails in Iowa? What are you going to say that hasn't been said?"

Many establishment Republicans, however, don't believe going two-for-two to start the nomination fight would put Trump on a glide path to the nomination.

"Folks are getting ahead of themselves," said Henry Barbour, an RNC committeeman from Mississippi who hasn't endorsed a candidate but believes that nominating either Trump or Cruz would cost the GOP the general election. "It takes 1,237 delegates to win the nomination and only 30 at stake in Iowa—long ways to go."