After months of campaigning during which Trump and other political outsiders have ripped up the script, voters in this midwestern state filed into church basements, school gymnasiums and libraries to have their say in the caucuses.
The contests in both parties are tight.
Trump has led the Republican polls for months, savaging early favorite Jeb Bush, and putting the rest of the large field -- 12 in all -- on the back foot.
But the controversial billionaire now faces a stiff test from ultra-conservative Senator Ted Cruz, who has invested heavily on the ground, hoping to get an early lead in the months-long race for the party's nomination.
"What a bit of history we are seeing," Cruz told fired up supporters on Monday.
However, political upsets are commonplace in Iowa: Will an establishment Republican -- Senator Marco Rubio, perhaps -- confound the polls and make it a three-way race?
For the Democrats, Clinton is looking to lay to rest the demons of 2008, when she lost in Iowa to now-President Barack Obama, and pursue her quest for history by dealing a solid blow to her upstart challenger Bernie Sanders.
"I know how to do this and I'm ready," a confident Clinton, vying to be America's first female president, told CNN hours before voting.
"There is just a lot of excitement and energy. I'm urging everybody to come out and caucus tonight to be part of this unique American process."
At most, a few hundred thousand people are expected to attend, highlighting the importance of voter turnout in an Iowa race where success can be the bedrock to a candidate's long-term success.
For both parties, most caucus meetings began at 7:00 pm (0100 GMT), and results are expected within hours. Such meetings to select candidates for each party take the place of primaries.
Getting out the vote
But it is not just Iowa that is up for grabs -- so too is the early momentum candidates are craving in the presidential race ahead of next week's primary in New Hampshire.
Clinton is under intense pressure to win here before going back to New Hampshire where Sanders, who is from neighboring Vermont, holds a strong lead.
Sanders, who describes himself as a democratic socialist, declared his race with Clinton a "toss-up."
A Quinnipiac University poll of likely Iowa voters released Monday agrees, showing Sanders even edging in front of Clinton, 46 to 43 percent -- within the margin of error of 3.2 percentage points.
Pollsters said Sanders' success would depend on turnout among first-time caucus participants.
Sanders has energized young Democrats with his denunciations of the "billionaire class" and his calls for a political revolution.
"If the turnout is high, I think we've got a real shot to win this," Sanders told CNN's "State of the Union" program.
Armies of campaign volunteers have fanned out through the state in recent days, knocking on doors or manning phone banks to get out the vote, while candidates dominated the air waves with talk show appearances and a relentless barrage of campaign ads.
"I never felt like I could get behind a candidate before Senator Cruz," says Nancy Anderson, who has been volunteering for his campaign since October.
Throwing a possible wrench in the works, a major winter storm was bearing down on Iowa, threatening snow and a dangerous wintry mix just as residents are caucusing.
Upsets possible
The Quinnipiac poll gave Trump a seven-point lead over Cruz, 31 to 24 percent, with Rubio in third at 17 percent.
But the real estate tycoon has recently downplayed his claims that he will win Iowa and he said Monday that he is not concerned with the see-sawing polls in the Hawkeye State, which are notoriously unpredictable.
"Today we have our ultimate poll," Trump said.
Both courted evangelicals in Iowa, who are expected to play a huge role -- they comprised 57 percent of caucus voters in 2012.
Rubio's star has meanwhile risen in recent weeks.
If he does well, the Florida senator could emerge as the top mainstream Republican heading into the first-in-the-US primary, to be held February 9 in the northeastern state of New Hampshire.
Long-shot Republican hopefuls like Carly Fiorina reminded voters that polls are notoriously unreliable in Iowa, something which will be of solace to Bush, who has fallen off in the polls and is barely part of the conversation.
