An unusually crowded field of Democratic presidential contenders has campaigned for months to take on President Donald Trump in November.
But before the hopefuls get there, they will have to win their party's nomination — and a crucial step to doing so, the Iowa caucus, takes place Monday.
To understand the significance of Iowa, it helps to look ahead. The Democratic nominating contest will end in July at the Democratic National Convention in Wisconsin.
The nominee named at that convention will be the Democrat who was won a majority of delegates during the caucuses and primaries that are hosted by each state, territory and the District of Columbia through June.
By tradition, the first contest is in Iowa, which gives the small Midwestern state outsized influence in selecting presidents. The state was crucial, for instance, to former President Barack Obama, whose victory there in the 2008 caucus helped propel him to victory over Hillary Clinton, his future secretary of State.
Nearly all of the major 2020 candidates — including former Vice President Joe Biden, Sens. Bernie Sanders, Elizabeth Warren and Amy Klobuchar, and Pete Buttigieg, the former mayor of South Bend, Indiana — have devoted substantial resources and time to winning the state.
The only exception, former New York Mayor Mike Bloomberg, entered the race late and is skipping the caucus entirely to focus on the big states that vote later, such as California and Texas.
Here is what you need to know about the Iowa caucus.
Caucus locations open no later than 6:30 p.m. CST. In order to participate, caucus voters will have to be in line by 7 p.m. CST.
The results of the Iowa caucus are expected to be released throughout the evening. Sometimes, the winner of the caucus is not known until the next day.
The results will be available on CNBC.com and CNBC's social media feeds.
Caucuses are like neighborhood party gatherings. They are held in schools, community buildings and churches around the state, and begin with messages from state and local party officials.
Instead of voting for a candidate using a secret ballot, caucuses involve physically moving to a designated part of a room along with others who support the same contender. Because it all happens in the open, Iowans are able to try to persuade others to switch their support.
Iowa has 1,678 precinct caucuses plus nearly 100 "satellite" caucuses for Iowans scattered around the country, along with some taking place internationally.
The voting process takes place in two phases.
After the second phase is done, support for each candidate is tallied again. The number of delegates each candidate receives is ultimately based on the number of supporters each candidate has once the nonviable contenders are eliminated.
This is what one caucus in 2016 looked like in a rural part of the state:
The Iowa caucus is important because it is the first time that voters around the country see how the candidates fare in a real contest, rather than just in polling averages.
The state's actual delegate prize is trivial: The viable candidates will split 41 pledged delegates, or just over 2% of the approximately 2,000 needed to win.
But the state's voters do tend to pick the same candidate who eventually wins the Democratic nomination. In fact, the last Democrat to win the Iowa caucus but lose the nomination was an Iowan: Sen. Tom Harkin, who won the caucus in 1992 after his rivals did not seriously contest it.
Bill Clinton, then governor of Arkansas, eventually won the party nomination while Harkin dropped out in March.
Iowa is not as predictive of the eventual president or of Republican nominees. Only former Presidents Jimmy Carter and Obama have won the presidency after winning the Iowa caucus in cycles dating to 1972.
Clinton remains the only person in the modern era to have won the presidency despite losing both the Iowa caucus and the New Hampshire primary, which is traditionally the second contest of the race.
Among Republicans, the winners of the last three Iowa caucuses have failed to secure their party's nomination. Sen. Ted Cruz of Texas won in 2016, former Sen. Rick Santorum of Pennsylvania won in 2012 and former Arkansas Gov. Mike Huckabee won in 2008.