Des Moines, Iowa (CNN)It's time to find out whether a movement can topple months of organizing and deep party ties.
Iowa caucuses: 5 things to watch
As Iowa voters caucus Monday night, both Donald Trump and Bernie Sanders will try to draw enough new voters into the process to defeat foes who have spent more money, devoted more time to organizing their support in the Hawkeye State and are backed by better-funded outside organizations.
Here are five things to watch:
The key to the Democratic race is no secret. If turnout looks like 2004's 124,000, Hillary Clinton almost certainly wins. If it's closer to 2008's 239,000, Vermont Sen. Bernie Sanders is the likely victor.
Clinton's Iowa operation has long focused on committed Democrats -- those who reliably caucus, many of whom supported her 2008 campaign.
Sanders, meanwhile, has focused his energy on college towns and has pushed hard to bring new participants into the caucus process.
Two counties where Sanders must run up a huge delegate advantage are Johnson, where the University of Iowa is located, and Story, the home of Iowa State University.
If he loses either -- or Clinton even comes particularly close -- Sanders won't win Iowa.
Polk County, the home of Des Moines and the state's most delegate-rich area, looks more like Clinton's territory: It's the state's largest urban area, and she has campaigned heavily both in its downtown and suburban regions. African-Americans and Latinos, small in number as they are in Iowa, could be the key to a Clinton victory.
There's another candidate hoping for an expanded electorate: Donald Trump.
The Republican front-runner's appeal to disaffected conservatives -- and some independents and "Reagan Democrats," too -- will be tested immediately. If he succeeds in drawing a huge turnout, it's a positive sign for Trump long past Iowa.
Texas Sen. Ted Cruz needs to win -- and win with a sizeable margin -- in the 44 counties west of Des Moines, which are more rural and more socially conservative than much of the state.
The establishment-leaning portion of the state could be Polk County and its more heavily-populated neighbors like Story and Dallas.
Northeastern and Southeastern Iowa were Ron Paul territory in 2012. Those areas are more libertarian than conservative, and could be the difference between Trump and Cruz.
Cruz turned Iowa into a boom-or-bust state when he predicted that Trump might run the table if he wins the Hawkeye State and told voters that caucusing for anyone else amounts to supporting Trump.
That strategy might push him over the top and into first place. But it also means anything else is a bust -- and could leave him limping out of the Hawkeye State into New Hampshire, where Trump is also in the lead.
Monday is also Trump's first time on the ballot. He has consistently boasted about his poll numbers while on the stump. How will he react should he lose, even if only by a couple of points?
Then there's what Twitter users are calling #Marcomentum: The sense that Florida Sen. Marco Rubio is surging late. Polls show him consistently running third behind Trump and Cruz, and if he manages to come close to those two, it would be a boon among establishment Republicans who are still searching for a candidate with a real path to the nomination.
Early-state results won't just identify front-runners -- they'll winnow what's an especially packed field of contenders on the Republican side.
If former Arkansas Gov. Mike Huckabee and former Pennsylvania Sen. Rick Santorum, the last two winners of the Iowa GOP caucuses, can't escape the low single digits, they could conclude they have no path to the nomination and drop out -- a move that would likely help Cruz, who appeals to a similar set of voters.
The three governors looking to win the "establishment lane," meanwhile -- Jeb Bush, Chris Christie and John Kasich -- are all looking ahead to New Hampshire. While a stronger-than-expected result in Iowa could help, a poor showing is unlikely to hurt enough for any to drop out.
On the Democratic side, Sanders would likely benefit most if former Maryland Gov. Martin O'Malley drops out. But he'd have preferred that it happen before Iowa, the state where O'Malley's support might matter most.
The final days in Iowa have been packed with surprises -- starting with Trump's decision to skip the final debate on Fox News.
Clinton was the subject of tough headlines headed into the weekend as the State Department refused to release 22 of her emails, retroactively classifying them -- and raising the specter, once again, of a lengthy, damaging federal investigation into her private email use during her tenure as secretary of state.
O'Malley's backers also have a choice to make. Democratic Iowa caucus rules say that to reach "viability," a candidate must have support of 15% of the caucus-goers in that precinct. If too few voters support a candidate, those individuals are freed to support another candidate of their choosing.
The former Maryland governor, however, isn't close to 15%. He's sitting at 3% in Saturday's Des Moines Register/Bloomberg Politics poll. His supporters could make the difference for Clinton or Sanders.
There's also the great unknown that Nature can deliver: A snowstorm is due in Iowa, and while it appears it will hold off until Tuesday, bad weather on caucus night would likely mean more people stay home.