Donald Trump is starting to build a serious campaign. The GOP is hitting back.

In Iowa on Tuesday night, Republican presidential frontrunner Donald Trump is speaking before an expected crowd of thousands at the Grand River Center in Dubuque, Iowa, drawing audiences his GOP competitors can only dream of at this point in the race. And, Reuters reports, Trump's increasingly sophisticated campaign will be ready to sign up those fans and "gawkers" to be precinct captains, campaign organizers, and other Trump foot soldiers in the Iowa caucus.

Trump's Republican rivals and the GOP establishment are starting to consider the real estate mogul a serious threat, and they are reacting accordingly. Early presumptive frontrunner Jeb Bush is starting to hit back at Trump's barrage of insults, The New York Times reports, stepping out of his comfort zone to call Trump's proposals "unrealistic" and suggesting he read his books, even while conceding that Trump is "a serious candidate."

And as Trump is making a serious play to win primaries and caucuses in early states, some state Republican Party committees are considering new hurdles Trump would have to jump to get on the ballot, Politico says. The two states mentioned are Virginia and North Carolina, each set to vote on proposals to make all official candidates pledge to support the GOP's eventual nominee. "The procedural moves are clearly aimed at Trump, who pointedly refused to rule out a third-party run during the first GOP debate," notes Politico's Alex Isenstadt.

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