With just four days to go until the Iowa caucuses, and polls showing relatively tight races in both parties, attention is now turning to which candidates will have the best get-out-the-vote operations. That's because what matters in Iowa is turnout.

Just ask Vermont senator and Democratic presidential candidate Bernie Sanders, who's lately seen his Iowa poll numbers register within one point of chief rival Hillary Clinton. “We will win if the turnout is large,” Sanders said at a Bloomberg Politics breakfast Thursday in Des Moines. “If the turnout is not large, we’re going to be struggling.”

A look at recent cycles shows a definite trend among registered Democratic voters to actually come out on caucus night, a plus for Sanders. Yet the bar set by then-candidate Barack Obama in 2008, when almost 240,000 Democrats participated, will be a very difficult one to meet.

On the Republican side, past cycles' candidates have struggled to energize the base and fill caucus halls. Turnout has hovered around 20 percent of registered party voters for the past two presidential election years, according to groups that track the statistics—compared to 40 percent for Democrats in 2008.

Iowa Governor Terry Branstad, a Republican, expects all that to change this year. “I think it's going to exceed any turnout we've ever had before,” he said in an interview with USA Today a few days ago. As for who he thinks is most likely to benefit from a surge in new Republican caucus participants: Donald Trump.

The University of San Francisco professor and Bloomberg Politics contributor Ken Goldstein concurs. “If you think the Iowa caucus electorate is going to be a bit over 120,000 people and have the composition that it typically does, then Cruz will likely win,” he said. “If you think that both the size and shape of GOP caucus participants is likely to be larger and different, then Trump will likely win.”

Before it's here, it's on the Bloomberg Terminal. LEARN MORE