"I've resisted the idea that Donald Trump could and would become the Republican nominee," writes GOP strategist Alex Castellanos in an email assessment of the presidential race. "Unhappily, I've changed my mind."

Castellanos -- who once said flatly that "Trump is not going to be the nominee" -- writes "the odds of Trump's success have increased and been validated in the past few weeks."

The key indicator, Castellanos says, is the fact that Trump dipped in the polls and now appears to be rising again. "In my experience, that tells us something important," Castellanos explains:

Republican voters went through a period of doubt about Trump, an understandable window of buyer's remorse. They went shopping for someone else -- but returned, finding no acceptable alternative who could match Trump's bad-boy strength and his capacity to bring indispensable change…Fearing they have only one last chance to rescue their country, they found no one else as big as their problem.

"In my experience, once voters doubt but return, doubting again is less likely," Castellanos concludes. "A candidate's vote hardens."

Castellanos, who played a key role in Mitt Romney's 2008 campaign, believes either Trump or Ben Carson will win the Iowa caucuses. If it's Trump, Castellanos sees him going on to win New Hampshire and then the nomination. Even if Carson wins Iowa, Castellanos sees a strong chance of Trump winning New Hampshire and then going on to take the nomination.

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