It all starts in Iowa. The first test of the 2024 presidential election arrives in Iowa on Jan. 15. That's when Republicans will meet to decide their preferred candidate for president, following the tradition of small local gatherings where neighbors and strangers can stand up to show their support for a particular candidate and persuade others to join them. Iowa Democrats will also meet that night, but to conduct party business only, not to decide presidential preference. Instead, picking a presidential nominee will be done by mail. The real impact of the Iowa caucus is momentum. Failing in Iowa means presidential hopefuls will need to take a hard look at their candidacy, and a weak showing by a front-runner could change the course of the election.Below, we take a look at the history of the Iowa caucuses over the past 20 years. How often has the state predicted the party’s eventual nominee? Iowa has a spotty record at picking the president, particularly for Republicans. Check out the interactive maps below to see a county-by-county look at results for the past six caucuses. Do the winners usually win the party's nomination?Not necessarily. In 2020, President Joe Biden didn’t win either Iowa or New Hampshire, but his campaign got a reset in South Carolina, and he went on to the Democratic nomination and the White House.In 2016, Donald Trump lost Iowa but won New Hampshire, similar to fellow Republican Mitt Romney in 2012.In 2008, Democrat Barack Obama won Iowa but lost New Hampshire. Republican John McCain lost Iowa that year but won New Hampshire.The last candidate to win both in Iowa and New Hampshire and go on to the White House was Democrat Jimmy Carter in 1976, although he technically placed second in Iowa, behind “uncommitted.”