Trump shows no signs of slowing down in Iowa

In Iowa, Donald Trump is the first choice among 23 percent of likely Republican caucus goers -jumping from 4 percent in May, according to a Bloomberg/Des Moines Register poll this weekend.

In a surprising surge to second, retired neurosurgeon Ben Carson is polling at 18 percent.

Republican voters appear to be warming to Trump's unconventional and confrontational style - his favorability numbers among Iowa Republicans have jumped 35 points since January.

And even some of his critics concede, he's becoming a better candidate.

"This is a movement, folks. This is not like maybe even about me. I don't want it to be about me. This is about common sense; it's about doing the right thing," Trump said Saturday in Nashville.

The surprising staying power of Trump's candidacy has left Jeb Bush, Marco Rubio, Ted Cruz and Scott Walker struggling to adapt - all are polling near the bottom in Iowa.

Bush is trying a new line of attack, hitting him on specific policies, like the frontrunner's immigration plan.

"It's not practical. And It's not conservative," the former Florida governor told supporters last week at a rally in Texas.

Trump has refused to rule out running independently if he doesn't get the nomination. But Saturday in Nashville, he seemed less concerned with a plan B.

"The Republican Party has been treating me very very fairly. All I ask is fairness. I'm leading in every poll by a lot. I'm leading in every state by a lot. A lot of things are really happening. In terms of victory, that would certainly be the best path."

One top Republican says they key for candidates as they take on Trump is to avoid questioning his legitimacy, while having a long-term plan to deal with him, because its looking like the summer of Trump could stretch well into the fall.

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    Julianna Goldman is a CBS News correspondent based in the Washington bureau.