Donald Trump heads into the Iowa Caucus with a 5-point lead over Texas Sen. Ted Cruz, while Florida Sen. Marco Rubio is a distant third. Donald Trump has 28 percent, followed by Cruz with 23 and Rubio with 15 percent.

Trump’s lead of 5 points is a reversal of a Register poll in early January. In that poll, Trump trailed Cruz by 3 points. Since then, Trump has gained 6 points, while Cruz has lost 2 points. Outside of the two candidates, however, there is very little change in the rest of the field.

Marco Rubio has gained 3 points over the last three weeks and is currently third with 15 percent support. Ben Carson has dropped a point and is 4th with 10 percent. Rand Paul and Chris Christie are at 5 and 3 percent respectively, with no change since the beginning of the month.

Jeb Bush, however, has lost half his support since early January. He is currently last with 2 percent support. Three weeks ago Bush had 4% support.

The race looks to be a very tight contest between Trump and Cruz. Supporters of both campaigns can find items in the final Register poll to make them optimistic. For Trump supporters, its a question of turnout. For Cruz supporters, it is the impact of evangelicals on the race.

The Register poll anticipates a turnout similar to 2012. Ann Selzer, the legendary Iowa pollster who conducted the poll, says she doesn’t see any indication of a huge turnout on Monday for Republicans. The number of first-time caucus-goers is largely in line with past elections.

If Selzer is wrong about this, the poll could be underestimating Trump’s support. He polls best among voters who haven’t participated in the caucus in the past. The 2012 caucus was the highest ever for Republicans, but if turnout is higher than that, it should boost Trump.

Perhaps more importantly, though, is that the Register poll may underestimate the number of evangelical voters. The poll expects evangelicals to make up 47 percent of those attending the caucus. This would be down considerably from both 2008 and 2012.

In 2008, evangelicals made up 60 percent of Republican voters, while in 2012 they accounted for 57 percent of caucus-goers. Selzer said she ran a poll model using an evangelical number closer to the 2012 election. In that case, the contest between Trump and Cruz was essentially tied.

Cruz leads Trump among evangelicals by 33-19 percent.

Another positive note for Cruz supporters is that in a head-to-head hypothetical match-up, Cruz beats Trump by a solid margin among Iowa Republican voters. Asked who they would rather see as the nominee, 53 percent of Iowa Republicans picked Cruz, while just 35 percent picked Trump.

Cruz, however, has been losing support over the past two months in Iowa. There has been a 15 point swing away from Cruz and towards Trump since early December. In that Register poll, Cruz led Trump 31-21. Since then, Cruz has lost 8 points and Trump has gained 7.

The Register poll was conducted Tuesday through Friday. While the timeline includes the Fox Debate Thursday, Selzer said she didn’t detect any impact on the poll. This isn’t surprising, as debates usually take a few days to show up in statewide poll results.

The poll did ask about Trump’s decision to boycott the debate, which was announced as the poll began. A plurality of Republicans, 45 percent, said the decision didn’t matter to them. Twenty-nine percent said they disapproved of Trump’s boycott, while 24 percent said they approved of the decision to skip the debate.

The final Des Moines Register poll has historically been one of the more accurate of the surveys of the caucus. The poll correctly predicted wins by Barack Obama and Mike Huckabee in 2008. Huckabee’s win was not unexpected, but the Register poll was the only survey to come close to predicting his 9-point victory margin.

The Register poll did miss Rick Santorum’s 2012 victory in the caucus, however.

Its final poll predicted that Mitt Romney would win the caucus by a 2-point margin. While that is within the poll’s margin of error, the Register showed Rep. Ron Paul running second. The eventual winner, Rick Santorum, trailed Romney in the final poll by 9 points.

The variable that powered Santorum’s surprise victory in 2012 and Huckabee’s larger-than-expected win in 2008 was the strength and size of turnout among evangelical voters. In 2008, 60 percent of Republican caucus-goers described themselves as evangelical.

The Register missed Santorum’s surge and underestimated Huckabee’s victory largely because it didn’t correctly predict the number of evangelicals taking part in the caucus.