Will Ted Cruz or Donald Trump win the Iowa caucuses? It depends on who shows up.

According to the most recent polls of Iowa, Donald Trump narrowly leads Sen. Ted Cruz (Texas) in the first-in-the-nation vote for the Republican presidential nomination. But who actually wins the caucuses will be determined by how many voters show up, and how conservative and/or religious they are. Trump is leading among first-time likely GOP caucus-voters, with 39 percent, according to a recent Wall Street Journal/NBC News/Marist poll, and if those new, generally more moderate Trump voters turn out, it's good news for the billionaire real estate mogul. "The bigger the turnout, the better it is for Trump," said Iowa Gov. Terry Branstad (R).

It's not just numbers, though. If the 2012 GOP Iowa caucus electorate shows up — nearly half of whom identified as "very conservative" — it should be a close race between Trump and Cruz. The Wall Street Journal predicts a much more moderate group, which bodes well for Trump.

On the Democratic side, Sen. Bernie Sanders (I-Vt.), like Trump, outperforms with first-time caucus-goers, so Sanders predicts that a large turnout on the Democratic side will propel him to victory over Hillary Clinton. A record 240,000 Iowa Democrats turned out in 2008, helping Barack Obama beat Clinton, but Clinton leads Sanders by three percentage points in the most recent Iowa polls.

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