01/30/2016 16:03 EST | Updated 01/30/2017 00:12 EST

Iowa caucuses: The history, the purpose, and why Donald Trump wants good weather

WASHINGTON — Tens of thousands of Iowans will put on their winter coats after supper Monday, wander out to public buildings across the state, and begin the process of picking a U.S. president.

The candidates they select aren't guaranteed the presidential nomination — far from it. Iowa's top finishers merely divvy up a few dozen delegates among the thousands who'll choose the Republican and Democratic nominees.

Those meagre statistical stakes help explain why Iowa has such a spotty track record at picking presidential nominees: less than a third of Republicans who win the state vote go on to take the nomination, while it's been a better-than-average barometer for Democrats.

But Iowa's political veterans say that's not the point.

Over at the old radio station where Ronald Reagan worked, the host of a political talk show argues that without this quirky tradition some recent U.S. presidents would never have come within sniffing distance of the White House.

Simon Conway says the tiny midwestern state provides two things. One is a chance for long-shot candidates to communicate with voters, without huge advertising dollars.

''I truly believe Barack Obama would not be president without Iowa,'' said the 1040 WHO host, referring to Obama's shocking 2008 caucus win.

''I actually don't believe Jimmy Carter would have been president without Iowa either.''

The other historical effect has been to cull the herd of candidates. Since 1976, every presidential nominee has either won Iowa or the following week's primary in New Hampshire — with one exception (Bill Clinton in 1992).

That herd-thinning dynamic explains why, despite being third in the polls, Sen. Marco Rubio is being bombarded by negative ads and pounding back: such second-tier candidates are competing to inherit the spoils as weaker rivals drop out, freeing up their supporters and financial donors.

The main contest on the Republican side appears to be billionaire Donald Trump versus conservative firebrand Ted Cruz. The milder-than-normal weather forecast could play a role.

That's because Trump is relying on first-time voters to show up. He leads the polls, thanks partly to supporters who've never voted in caucuses before. Cruz has an edge among the well-organized, high-turnout evangelical Christians.

Less than 20 per cent of eligible voters participated last time.

''I don't know if (Trump supporters) show up,'' Conway said. ''He gets big crowds, and if those turn into actual people at caucus, he could win.''

Caucusing requires more dedication than a regular election: It's not just the cold voters must endure, but also a lengthy event featuring multiple speeches. Trump's daughter Ivanka posted a video on social media explaining the process to supporters.

The events start at 7 p.m. statewide. People will gather in libraries, churches and community centres to listen to speeches from the candidates' supporters.

Republicans will scrawl the name of their preferred candidate on a piece of paper and submit it. It's different for Democrats. People will gather in different parts of a room.

Eventually, a moderator will ask supporters of the weakest Democrat — likely ex-Maryland Gov. Martin O'Malley — to disperse and join either the Hillary Clinton or Sen. Bernie Sanders crowds.

Votes get tallied up on smartphone apps and sent to state officials.

"It's about as grass-roots a type of event as you can get,'' said Greg Greiner, a national-security lawyer and ex-Iowa county Republican co-chair.

''It's literally people just finishing up their dinner, putting on their coat, going out and giving their voice."

The tradition emerged from the violent clashes over Vietnam outside the 1968 Democratic nominating convention, says veteran Iowa political writer David Yepsen.

The process was democratized in 1972, with power shifting from party bosses on the convention floor. With a state-level caucus tradition already, Iowa Democrats set an early date to allow time for voters to pick delegates to state conventions, and for state delegates to the summer national convention.

The Iowa caucuses' legend was born when a little-known anti-war senator, George McGovern, finished a surprising second then won the nomination — a dark-horse triumph later replicated by Carter and Obama. Republicans adopted caucusing in 1976.

The tradition has changed.

Yepsen recalls the intimate events of yesteryear. He remembers being the only journalist at a Carter event in 1976. And the time he and Joe Biden borrowed a hotel clerk's car keys to get a late-night pizza — Biden drove the car.

''You could literally hop in the back seat of a car and drive across the state with a presidential candidate and a driver,'' said Yepsen, who spent 34 years at the Des Moines Register.

''You had all kinds of time to talk to them. Now these events are, they’re just huge inside. You’ll have hundreds if not thousands or people.''

The event has gotten so big, even people actively avoiding politics can't help notice the ads, crowds and posters.

One of those apolitical types is a Canadian student who coaches the University of Iowa's hockey goalies.

Scott Ismond hasn't sought the campaign. Just over a week ago, though, it came to him.

''I'm going to (hockey) practice and there's police cars around the rink,'' he said.

''You see lines of people outside.''

They were there for Trump, introducing his latest supporter: Sarah Palin.

The Canadian Press