DES MOINES, Iowa (AP) -- Since the 1840s, political activists in Iowa have gathered on dark winter nights to take care of party business. Only for the past 40 years has anyone outside the state taken notice of these Iowa caucuses.
Everything changed in 1972 when Democrats moved their meeting to January, making Iowa's presidential preference vote the first in the nation. Noticing the change, candidate George McGovern gave Iowa extra attention, then finished a surprising second and went on to win his party's nomination.
Republicans moved up their caucus in 1976, and candidates have paid rabid attention to Iowa ever since.
Politicians aspiring to the White House begin visiting Iowa to make connections with local politicians and activists months or even years before caucus night. Their hope is that by winning the caucuses -- or simply beating expectations -- they'll also win a burst of publicity that will help fundraising and improve their chances in New Hampshire and other early-voting states.
For Republicans, the nation's attention is technically all that's at stake Tuesday. While Democrats choose delegates who are expected to support the selected candidates at the party's national convention, Republicans handle that at county and district conventions later in the year. That means the GOP's caucuses are essentially a nonbinding straw poll.
The parties hold caucus meetings for all of the state's 1,774 precincts -- at schools, churches and even some private homes. Because some places hold multiple caucus meetings, Republicans will gather Tuesday night in about 800 locations in all 99 of the state's counties.
Anyone who so chooses can speak in support of a candidate at a Republican caucus meeting. Ballots are then passed out and participants privately mark their choices. Those ballots are counted and the results called into party headquarters, where they are posted online immediately.
People can register with the party when they arrive, and all those who will be 18 by the general election can participate. But the commitment to attend in person -- there is no absentee or early voting -- means the caucuses draw a relatively small slice of Iowa's roughly 3 million residents. Only about 17 percent of eligible voters took part in the record-setting turnout year of 2008.
That year, Barack Obama's repeated trips to Iowa paid off with a Democratic caucus win over Hillary Rodham Clinton and John Edwards. In the Republican caucus, Mike Huckabee finished first and Mitt Romney second. Eventual GOP nominee John McCain finished in fourth place, just behind Fred Thompson.
This year, the focus is entirely on Republicans, but there will be Democrats caucusing Tuesday. The crowds will undoubtedly be much smaller, since Obama has no competition for the Democratic nomination.
For those Democrats who do attend, the process is more complicated that the GOP's simple secret-ballot straw poll. Democrats break into preference groups at their caucus meetings, publicly declaring which candidate they favor. Candidates must get support from 15 percent of those attending to receive votes, and activists try to win over those whose candidates have fallen short of the 15 percent threshold.
The results are then reported to party headquarters, where they are run through a formula that changes the value of votes based on a county-by-county analysis of Democratic performance in the last gubernatorial and presidential elections.