A visitor casts their vote with a kernel of corn for presidential candidate Donald Trump in a straw poll at the Iowa State Fair, Aug. 20, 2015, in Des Moines, Iowa. 
Photo by Paul Sancya/AP

The key to Iowa: Evangelical voters

For the Republican presidential hopefuls in Iowa, the numbers from the last few caucuses hold a simple message: win the evangelical vote to win the state.

It was true in 2008 with former Arkansas Gov. Mike Huckabee and in 2012 with former Pennsylvania Sen. Rick Santorum. Those candidates captured the evangelical vote by double digits in the caucus and both men won the state.

So going by that simple formula, who has the GOP edge in the Hawkeye State? Texas Sen. Ted Cruz. In the latest NBC News/Wall Street Journal/Marist poll of the state, Mr. Cruz has 33 percent of the evangelical vote among likely Republican Caucus goers. Donald Trump is at 19 percent among those voters and Florida Sen. Marco Rubio is at 14 percent.

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The numbers are about more than an interesting correlation. The evangelical vote in Iowa is crucial for Republicans because it makes up a huge chunk of the Republican caucus vote. When the presidential field is as big as it is in 2016, a candidate doesn’t need to win a majority of evangelicals to win the state, he or she just needs to win them solidly.

In 2012, Mr. Santorum eked out a win in Iowa with evangelical voter support that looks a great deal like Mr. Cruz’s current support.

These numbers are not lost on Mr. Trump who has been trying to build his evangelical bona fides in recent days. On Monday the frontrunner in the national polls spoke at Liberty University, a Christian university in Virginia, quoting scripture and promising to protect Christianity.

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The real question going into Iowa is if Mr. Trump can peal away a few percentage points of Mr. Cruz’s evangelical edge in the state. The Trump campaign has done much to disrupt the status quo of the 2016 campaign, but its difficult to disrupt the power of the evangelical vote in Iowa.

It’s a force that he’ll likely need to harness if he wants to win there. 

This story originally appeared on NBCNews.com