On Monday night, Iowans will show up to 1,681 precinct locations across the states to caucus for their preferred Democratic or Republican presidential candidate.
It's a critical night that starts off the primary voting process and can have some influence on the rest of the race.
On the Democratic side, voters can choose from Hillary Clinton, Bernie Sanders and Martin O'Malley. On the Republican side, Iowans will be able to caucus for Donald Trump, Ted Cruz, Marco Rubio, Chris Christie, Jeb Bush, Rand Paul, Ben Carson, John Kasich, Carly Fiorina, Jim Gilmore, Mike Huckabee and Rick Santorum.
- Watch CBSN for updates on results from the Iowa caucuses
The caucuses officially begin at 7 p.m. CT or 8 p.m. ET in which voters show up at school gyms, churches and community centers. About 1,100 precinct locations will be open for Democrats and 900 for Republicans. Democrats are also expected to hold a "tele-caucus" for U.S. voters who live overseas as well as "satellite caucuses" at places like nursing homes and large businesses where people work an overnight shift.
Republican caucus goers will show up at a precinct, hear messages of support for a certain candidate and then cast a secret ballot. Delegates for the Republican National Convention are awarded proportionally based on the tallies statewide.
It's a bit more complicated on the Democratic side because there's no ballot involved. Democratic caucus goers have to physically stand with other supporters of their preferred candidate and, in order for candidates to win over delegates, they must have at least 15 percent of attendees in their corner.
Results from both Democratic and GOP precincts will be reported by a new mobile app the parties created with Microsoft and Interknowlogy.
The outcome of the caucuses are up in the air, with recent polls showing tight races between Clinton and Sanders as well as Trump and Cruz. Then-candidate Obama defeated Clinton at the Iowa caucuses in 2008, which was a breakthrough moment for his campaign.
The latest CBS News battleground tracker surveys released last weekend found 47 percent of likely Democratic caucus goers would back Sanders compared to 46 percent who would support Clinton. For likely GOP caucus goers, 39 percent said they would back Trump and 34 percent said they would support Cruz.
Trump decided to skip the final GOP presidential debate before the caucuses on Thursday and held a fundraising event for veterans in Des Moines instead.
Tensions between the Clinton and Sanders campaigns, meanwhile, have been heated for weeks. In mid-January, the Vermont independent said that Clinton's campaign is getting "very, very nervous" because support for him had been picking up in the polls. Sanders has predicted that a heavy turnout would benefit him at the caucuses because he has attracted a lot of first-time voters. The same applies to Trump, who has attracted the same type of voters.
The Iowa caucuses are not always critical to securing the nomination. In 2008, for example, Huckabee won the caucuses, but Sen. John McCain went on to win the nomination. In 2012, Santorum won the caucuses with a slim 34-vote margin that took days to finalize. Second-place finisher Mitt Romney went on to become the Republican nominee.
→ What: Iowa caucuses
→ Where: Across Iowa
→ When: 8 p.m. ET on Monday
CBS News' Rebecca Kaplan contributed to this story.
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