NPR's live blog The Stream will bring you live news, photos and analysis from NPR's political team on the ground in Iowa and in Washington, D.C. It will open at 4 p.m. ET and be updated all night.
Get Caught Up:
What we're watching: Turnout. Caucusing is harder than voting in a primary. Both Donald Trump and Bernie Sanders lead by double digits with people who tell pollsters they will be participating in their first caucus. The big questions if all those people lining up for their rallies will turn out to caucus.
What's a caucus? Iowans gather in neighborhood meetings, where they speak on behalf of their candidates and try to persuade others to support him or her.
How it works: Democrats cluster by candidate and are then counted. A candidate who receives less than 15 percent of the support in the room is considered not viable in that caucus. Those supporters have to choose another candidate. Republicans vote by secret ballot.
Who cares? Who wins these caucuses matters, especially for momentum. Winning can help propel a candidate to the White House — like Barack Obama in 2008 or George W. Bush in 2000. Losing badly can, and very likely will tonight, lead several contenders right to the dropped-out-candidates' club. But it's only the beginning and certainly isn't always predictive.
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