Presidential candidates Donald Trump and Hillary Clinton have maintained steady leads for months in the race for the nominations of their respective parties. Monday, however, might well be the moment of truth, as Iowans weigh in on the 2016 presidential race.

It perhaps seems strange that a state as small as Iowa has become so crucial to the U.S. presidential elections. But Iowa, as the first state to vote, offers a hint into American public opinion and in the past has proved significant. President Barack Obama saw his popularity soar after the Iowa caucuses in 2008, and John Kerry also came out of nowhere in Iowa in 2004. If pre-Iowa polling is accurate, several Republican candidates could end their campaigns, and that could give another candidate the boost needed to get ahead of Trump — or it could strengthen Trump's lead.

The event started as an outcome of the contentious 1968 Democratic National Convention in Chicago. The Democratic establishment was criticized for its power over the nomination process and selection of convention delegates. The party responded with new rules for picking delegates to modernize the process, and changed the calendar date of its caucus system in Iowa from May 20 to Jan. 24, making it the first state to vote in the Democratic primary election in 1972. 

But it was Jimmy Carter’s campaign in 1976 that really revolutionized the Iowa caucuses. He was at the time a little-known governor who campaigned across the state, appearing on radio and television. All the attention helped propel Carter forward to the nomination and encouraged other candidates to focus on their campaigns in Iowa. 

“Iowa became super important because we — the media, party insiders, activists, the candidates themselves, and even voters to an extent — gradually decided to make  it so important,” Vox wrote.

Due to all the attention surrounding the Iowa vote, the Republican Party also decided to switch dates in 1976. Since then, the caucuses have only grown in importance.

The Iowa caucuses require individuals to actually show up for an in-person event at one of 1,681 precincts in the state. Each party administers its own caucuses and Democrats and Republicans have different rules.

The GOP’s guidelines are considerably simpler than the Democratic ones. After some rallying from precinct captains or the presidential candidates themselves, attendees write down their votes on paper and turn them in to be counted statewide.

The Democratic caucuses, on the other hand, are more complicated, hourslong events. Voters group into sections designated for their preferred candidates and are then counted. Candidates who fail to get 15 percent of voters are disqualified and the caucus captains then lobby to have others join their side.

As of Friday, Trump was in first in Iowa with 32 percent, with Texas Sen. Ted Cruz in second at 25 percent. Florida Sen. Marco Rubio polled at 14 percent. On the Democratic side of the race, Clinton polled at 47 percent and Sen. Bernie Sanders at 45 percent, NBC News reported.