Trump strong

U.S. Republican presidential candidate Donald Trump speaks to supporters at the Westin Hilton Head Island Resort and Spa in Hilton Head Island, South Carolina, December 30, 2015. REUTERS/Randall Hill

This article originally appeared in Slate.

Donald Trump made his fortune building commercial real estate, but as a candidate he's selling a gated community. The Republican front-runner released his first ad Monday, and its message is that he will keep them out. By them, he means Mexicans and Muslims. The ad is not subtle. It includes images of ISIS fighters, the San Bernardino shooters, and dark figures crossing the border in hordes like an army storming the keep. (For those of you who thought the cinematic quality of the invading army seemed almost too menacing to be true, it turns out you were right. According to Politifact, it wasn't people crossing the Mexican border. The footage in the Trump ad is of people crossing the Moroccan border, who pose no immediate threat to the United States, an ocean away.)

Donald Trump: I’m going to spend $2 million... 01:30

The ad is a distillation of the Trump pitch. It's a bouillon cube of Trump, the one message he wants voters to have in their minds when they go into the voting booth. Campaigns are about creating a need that only one candidate can fulfill. In this case Trump is the protector. Polls show that on the issue of terrorism,GOP voters trust Trump above all others. The ad is not about his ability to make great trade deals or increase wages. He's going to keep you safe. And if that wasn't already your concern--terrorism is the top worry for GOP voters--it might very well be after watching the ad.

Trump is promising to keep America from being overrun. That also means, according to his first political spot, to "cut the head off ISIS." That promise is an evolution from Trump's original passive rumination that it might be OK if the Russians took care of the terrorist group in Syria.

Rick Santorum also put out a new ad on Monday. "We are at war and it is too risky to elect another senator with just a couple years experience while the world is burning," says the ad in an obvious dig at Sens. Ted Cruz, Marco Rubio, and Rand Paul. It then goes on to tout the former Pennsylvania senator's experience in the Senate handling national security issues.

Santorum's ad seems almost quaint by comparison in its adherence to pre-Trump rules of politics. So far, GOP voters who have responded in polls haven't prized experience. Instead they have rewarded perceived strength and bluntness--campaign rhetoric and behavior that they believe will translate into strength in the Oval Office.

Extended interview: Donald Trump, January 3 18:08

Trump has none of the experience that Santorum is talking about. He doesn't even have the experience that any of the senators running have, but so far that has only helped Trump's candidacy. He's running on his success in the private sector, which he argues can be easily transferred to government.

It's not just the Trump ad that seeks to create an appetite in the electorate for strength. When it comes to policy debates, the front-runner keeps defining the contest in terms of weakness and strength. On Face the Nation he offered a perfect example of how he frames these issues for Republican primary voters on the issue of immigration:

Ted Cruz is trying to step up his whole game on amnesty and on illegal immigration, because it was actually quite weak. And you listen to him and Marco Rubio, they're trying to solve the problems that they have had in the past, because they were both weak on it. And I have been very strong on it. So they're trying to get stronger.

So far the GOP candidates have joined Trump in making the presidential campaign about strength against foreign threats. In doing so they have accepted that the contest will take place on the terms Trump finds most advantageous for himself. With less than 30 days to go before the voting begins, Trump would like to keep everyone on that turf and put a wall around it.