Long lines at Trump rallies show his potential to win the Iowa caucuses. But with opportunity comes a giant challenge: Many of those who flock to see the GOP candidate are not registered to vote, or they are not registered Republicans.
Trump aides walk the lines handing out registration forms that can be turned in caucus night, and the candidate implores his crowds to show up.
Whether they will is the huge question here.
About 120,000 Republicans voted in the 2012 caucuses. There is a general consensus among the competing GOP campaigns that a traditional turnout -- given the crowded field and high interest -- should be 130,000 to 140,000.
Jonathan Martin of The New York Times says every campaign acknowledges predicting turnout is a risky business, but some campaign staffers have said it will end up being a numbers game.
"If (turnout) is about 135,000 to 140,000, (Ted) Cruz aides say he will probably win. If it gets higher than 150 (thousand), toward 160 or 170, that's probably the danger zone for the Cruz folks and that's where Trump looks more like a winner."
A Democratic 'Trump effect' too?
Is Trump helping Bernie Sanders?
CNN's Jeff Zeleny explored the "Trump effect" on the Democratic race.
"For all the disruption that's going on on the Republican side, some is spilling over onto their side. If they think the regular order has been shaken up, why do they have to fall in line? Why do these Democrats have to support the establishment candidate? So there is a lot of belief on the Democratic side and (some are saying), 'You know what, let's go with Bernie Sanders.'"
Team Clinton hopes for an Iowa win, but looks toward South Carolina
It's hard not to escape the "deja vu" mood of Democrats in Iowa, where Sanders is mounting a late charge that threatens what for months was a big Clinton advantage in the state.
As Clinton's attacks on her rival escalate, Sanders has started reminding his supporters that it all seems familiar -- noting that in 2008 Clinton went after then-Sen. Barack Obama with similar lines about not being ready for the job.
Team Clinton hopes its strategy works here in Iowa. But Maggie Haberman of The New York Times told us there is also a belief that even if 2016 begins similarly to 2008, there will be at least one big difference this year.
That would be South Carolina -- the first state where the African-American base of the Democratic Party comes into play in a major way.
"The key here is that after Obama won Iowa (in 2008), that was when a lot of the black vote in South Carolina shifted to him. Bernie Sanders is not the same kind of candidate."
Iowa usually narrows the field and often brings a surprise
Yes, George W. Bush won here in 2000 and went on to be the Republican nominee. Ditto for Obama on the Democratic side in 2008.
But Iowa's traditional role is to winnow the presidential field, not pick the nominees. And with the GOP race so crowded this cycle, that could be a very important -- and consequential -- role.
There are two past Iowa winners in the GOP field, Mike Huckabee and Rick Santorum, and it is hard to see them pressing on if they can't climb from the bottom of the pack here. Ben Carson could also be a question mark; he was leading Iowa polls a couple of months ago, but has slipped into the middle of the pack now.
And then there is Paul.
The senator from Kentucky vows to be this cycle's Iowa surprise.
His team believes Paul's support is underestimated in the polls. Plus, it's counting on a not-so-secret weapon: Iowa's colleges are in session this caucus day. The last two caucuses were during winter break.
Students for Rand has a goal of turning out 10,000 students statewide. One organizer said the group had closer to 6,000 students who had signed up for the effort -- perhaps short of the goal, but still a healthy number.
And the Paul campaign this week is calling on another weapon: Ron Paul, the candidate's dad.
The Republican former congressman has been largely absent from the trail this year, but is headlining a caucus-eve rally next Sunday at the University of Iowa.
Ron Paul ran a strong third here in 2012, with 22% of the vote.
The Rand Paul campaign hopes he can energize his old network, as well as help his son energize the college students.
Team Paul says a strong showing here would send Rand Paul on to New Hampshire with a breeze at his back.
But if he falls short, he could face serious pressure to end his campaign.
His Senate seat is also on the ballot this year, and some Republicans back home and in Washington have suggested that Paul invest more time and energy on that race.
He has resisted, insisting he still has a path in the presidential race. Iowa's vote could help settle that debate.