Will Iowa drop the curtain on Trump's 'showtime?'

In his first big ad buy, Donald Trump this week doubled down on his plan to temporarily block foreign Muslims from entering the United States. The question now is whether the ad, which also invokes his promise to build a giant wall on the Mexican border (somehow paid for by Mexico), will reverse his second-place status in the looming Iowa caucuses and prevent a big and possibly crippling loss the first time Republicans actually cast votes.

Trump still enjoys big leads in national polls, including a new survey out on Tuesday that showed him with 35 percent support among registered Republicans to just 18 percent for Texas Sen. Ted Cruz and 14 percent for Florida Sen. Marco Rubio.

Donald Trump
Brian Snyder | Reuters
Donald Trump

But Trump remains stuck behind Cruz in Iowa, and some GOP operatives say barring a major turnaround, the real estate billionaire will get tagged with the label he hates most — loser — after the Feb. 1 caucuses.

A loss in Iowa would not necessarily derail Trump. He still has big leads over Cruz and Rubio in New Hampshire and South Carolina, the next two states to vote, and could quickly re-establish his front-runner status. But Trump's entire political brand is based on winning and domination. And the GOP primary process is highly fluid with the results in one state impacting the next. Trump's New Hampshire lead could vanish quickly depending on how he handles a possible loss in Iowa.

The post-Iowa moment will go a long way toward deciding whether the conventional wisdom — that GOP voters will abandon their angry protest support for Trump in favor of more traditional candidates — is accurate or just wishful thinking. If Trump loses Iowa but comes back to win New Hampshire and South Carolina, there is a real chance he will become unstoppable and capture the GOP nomination for president. That will be especially true if the field remains large heading into the Super Tuesday primaries across the South on March 1.

If Rubio, former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie and Ohio Gov. John Kasich all remain in the race, splitting up the more centrist, establishment vote, Trump will have a clearer path to the nomination assuming he can fend off Cruz and retired neurosurgeon Ben Carson for the conservative, evangelical base of the party.

Chances are fairly good that the field will in fact thin out before Super Tuesday and certainly after it. Bush has essentially gone all-in in New Hampshire and other early voting states, and if he does not finish strongly he may have little choice but to get out of the race. The same is probably true for Christie and Kasich.

If the field does shrink significantly after Iowa, New Hampshire, South Carolina and Nevada, Trump may wind up battling Cruz for the activist base vote and Rubio for voters more concerned with nominating a candidate viewed as electable against likely Democratic nominee Hillary Clinton.

The establishment wing is clearly banking on Republicans "getting serious" as actual voting nears and rejecting Trump as too radical in his immigration positions and lacking in deep knowledge on serious issues.

Christie played this card in New Hampshire on Monday, referring to Trump when he said "showtime is over" and adding that "we are not electing an entertainer-in-chief." And Trump gives those who question his seriousness significant fodder on a near daily basis. In an interview with Fox News' Bill O'Reilly on Monday, Trump struggled to explain how he would handle the current tensions between Iran and Saudi Arabia as president. Asked if he would support military action against Iran, Trump said he would want to help Saudi Arabia and then suggested the Saudis should be helping the U.S. economically.

Pressed by O'Reilly to explain his policy toward Iran and how far he would go to address its nuclear program, Trump basically punted. "Bill, I want to do what's right," he said. "I'm not gonna tell you right now what I'm gonna do." Asked if voters had a right to know whether he would consider war against Iran, Trump said "No, they don't."

It remains unclear whether Trump's bombastic approach that focuses on restoring American "greatness" while studiously avoiding or stumbling over policy details will continue to sell when voters actually start going to the polls. Iowa voters have the first chance to answer that question in less than a month. We will then finally find out if "showtime" is really over or just getting underway.

—Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.