The process of choosing a new US president begins every four years with the Iowa caucuses, a ritual seen by some as peculiar, but which is pivotal in shaping the field of White House hopefuls.
Voters in this largely rural midwestern state will gather late Tuesday in church basements, school auditoriums and even in some private homes for one of the US electoral calendar's quirkiest but most important electoral events.
Republican voters will make their choice of a nominee to oppose Barack Obama in the November presidential election, while Democrats in separate caucus gatherings will affirm Obama as their candidate for a second four-year term.
The lead-off state in America's election marathon since 1972, Iowa has what some critics say is an outsized role in determining the eventual nominees in November's general election.
The half-dozen Republican hopefuls vying for their party's presidential nomination all have spent considerable time in Iowa, in the months -- in some cases, years -- before Tuesday's caucuses to woo voters.
The state is among a handful to hold the more personal caucuses, rather than primaries. Viewed as "gatherings of neighbors," participants must publicly state their support for the candidate of their choice.
The ritual -- requiring that voters venture from home on a frigid winter's night to sit through hours of speeches about the candidates before casting their ballot -- ensures that only the most dedicated partisans take part.
But the process -- and the choice of Iowa to launch it every four years -- has its critics.
The heartland state had three million inhabitants, but only about one in five of Iowa's registered Republicans will take part in the caucuses. That translates to about four percent of the state's population.
Some critics point to the relatively small number of participants as a reason to hold the first presidential nominating vote in another state. The 2008 Republican caucus in Iowa drew fewer than 120,000 participants.
Others point to the fact that Iowa, which is more than 91 percent white according to US Census figures, is not representative of the broader American population.
The state is also more rural and more religious than the country as a whole, with about 60 percent of Republicans identifying themselves as evangelical Christians.
New York Times columnist Gail Collins last week urged her readers to "feel free to ignore Iowa" given the small number of voters who actually take part in the caucus.
"The Republicans hope to get more than 100,000 participants. That's about the same number of people in Pomona, California," she wrote.
"Imagine your reaction to seeing a story saying that a plurality of people in Pomona thought (former US House Speaker) Newt Gingrich would be the best GOP presidential candidate.
"Would you say wow, I guess Newt is now the front-runner? Possibly not," Collins wrote.
Despite the criticism, Iowans staunchly defend their key role in the presidential election process.
"I think Iowa is representative. If you look at the last four national presidential elections, Iowa's popular vote has mirrored what has happened nationally," Matt Strawn, chairman of Iowa's Republican Party, told NBC television over the weekend.
"You also have to think what our role in the process is. We're first. We're not last. We're not the decider. We start winnowing the field," he said.