The final Des Moines Register poll before the voting begins on Monday night shows Donald Trump with a narrow lead over Ted Cruz, 29-25%. Marco Rubio is 3rd with 15%. Ben Carson is 4th with 10%

The rest of the field is in the low single digits.

On the Democratic side, Hillary Clinton and Bernie Sanders are in a virtual dead heat. Clinton gets 45% of the vote with Sanders earning 42%. With the poll's margin of error at 4%, it appears that whichever candidate has the superior ground game and can generate the most enthusiasm to get their supporters to the Caucuses will prevail.

Trump stands at 28 percent, while rival Ted Cruz has slid to 23 percent. But there’s still a strong case for Cruz in this race — he’s more popular and respected than Trump, the final Des Moines Register/Bloomberg Politics Iowa Poll shows.

“The drill-down shows, if anything, stronger alignment with Cruz than Trump, except for the horse race,” said J. Ann Selzer, the pollster for the Iowa Poll.

Mainstream Republicans, faced with seeing governors Jeb Bush and Chris Christie stalling and the grim reality looming of a victory by a smash-mouth game show host or an ultra-conservative obstructionist, have gravitated toward Marco Rubio. The young-looking, first-term U.S. senator from Florida is now at 15 percent. Still, Trump gets more of their support.

“Donald Trump could win Iowa,” said Stuart Stevens, a Maryland-based GOP strategist who has worked on five presidential campaigns but is neutral this election cycle. “But he has little room for error. He is almost no one's second choice.”

In a primary, "second choice" is a pretty meaningless stat. But in a caucus, where attendees are allowed to change their vote, it could be crucial. 

Supporters of candidates like Chris Christie and Jeb Bush, who have no chance of showing well, might decide in the end to gravitate to the side of the room where Cruz or Rubio supporters have gathered. Rubio leads the "second choice" pack with 20% with Cruz a close second at 17%. 

Cruz will also benefit if there is a higher than expected turnout of evangelical voters where he does extremely well in the polls.

Another factor to weigh is that Cruz has a substantial advantage in favorabilty rankings among Iowa voters. Trump's numbers are underwater with a 44% favorability rating while Cruz stands at 56%. 

Could this translate into a narrow Cruz victory? Conventional wisdom says yes. But there are intangibles at work as well, with the biggest question mark being the Trump voter. Will they or won't they show? The result of the contest will probably rest on the answer to that question.