It's Jan. 3 in Iowa and you decide, what the heck, I'm going to a precinct caucus.

Not affiliated with a political party? Not registered? Not even old enough to vote?

No problem. Come and help choose the nation's next president.

In yet another quirk of Iowa's caucus system, all citizens can participate as long as they sign a voter registration card, attesting to residency in the precinct and show that they'll be 18 in time for the general election.

"It has not been a problem," said state Democratic Party spokeswoman Carrie Giddins.

Some people do have a problem with the ease of registering for New Hampshire's leadoff primary, which follows Iowa's caucuses by five days.

New Hampshire allows same-day registration at the polls, has no minimum residency period and defines a voter's home as the place where he or she sleeps most nights or intends to return after a temporary absence. The state, not the parties, runs the primary, and changes to residency laws have been hotly contested.

This year, New Hampshire Democrats pushed through a change that some Republicans contend would enable campaigns to bus in people who could cast a ballot and then vote again in their real home states.

"You can vote in New Hampshire without being a resident," said Republican state Sen. Bob Clegg. "You can vote in the primary because you someday may want to live here."

Democratic state Sen. Peter Burling calls such arguments "part of the campaign of fear to restrict people's right to vote."

David Scanlan, New Hampshire's deputy secretary of state, acknowledged the law is ambiguous about prohibiting people from voting in more than one state. But he insisted there are no widespread problems.

"Everybody has the right to vote somewhere," he said. "The question is where that place is."

In Iowa, requirements for taking part in the caucuses are becoming a focus for candidates now that the contest is a little more than a month away. The voter registration rule — or lack of one — is among a handful of unusual policies that make the caucuses puzzling even to many Iowans.

Campaigns have been trying to explain away the mystery in an effort to attract potential supporters.

Democrat Hillary Rodham Clinton has produced a video starring her husband, former President Clinton, to explain the caucuses process, and campaign officials hope up to 50,000 activists eventually see it. Rival Barack Obama has launched a "caucus pros" program, in which seniors and other longtime participants help newcomers know what to expect.

Republican Rudy Giuliani's campaign has held training sessions around the state, led by prominent folks like former pro football quarterback Fran Tarkenton, who played for the Vikings just up the road in Minneapolis.

The caucuses, run by the parties and not state election officials, have long allowed voters to show up and register or switch party registration. If they're found to have lied they can be fined up to $7,500, but problems rarely arise.

Giddins said the only incident in recent memory came in 2000, when an out-of-state reporter tried to register as a precinct resident. He was charged and paid a fine, she said.

If gaining access to your precinct caucus in Iowa is a low-key affair, actually participating can be far more volatile.

Unlike what most Americans are accustomed to, there's no voting booth — and nothing private about the 1,784 precinct caucuses held in church basements, fire stations and libraries.

"Discussions can happen, old wounds can flare again from activists and it can get intense," said veteran Democratic operative Matt Paul. "One mistake that people make is underestimating the value of neighbors looking at their neighbors and wondering what they are going to do. You have to publicly stand, physically stand in support of your candidate."

Although both parties welcome nearly anyone willing to venture out during a cold Midwestern night, they take vastly different approaches.

On the Republican side, activists at each meeting elect a leader, then backers of each candidate deliver speeches on their behalf. Those gathered then publicly vote, often by raising their hands but sometimes by marking a ballot.

After the results are phoned in to a central reporting system, activists turn to party business such as beginning to write a party platform, electing precinct officers and picking delegates to county conventions in March.

"Ours is fairly simple compared to the Democrats," said Chuck Laudner, executive director of the Republican Party of Iowa.

When Democrats gather, they elect folks to run the meeting and then turn to the issue of "viability." In essence, a candidates must have the backing of 15 percent of the people who show up at each caucus to be eligible to win any delegates and move to the next step. Once that number is determined, the activists break into preference groups for the candidates they favor.

That's when it gets interesting.

"A couple of things can happen," Giddins said.

Backers of candidates who don't reach the 15 percent threshold, can join with other groups to reach that magic percentage. They can also agree to join forces with a group that is viable. Such decisions come after negotiations over issues such as who will be elected as a delegate, or who will back a proposed platform plank later in the evening.

Caucus-goers then gather in candidate groups, and those numbers are counted and run through a formula that results in the number of delegates reported to a central tabulation area.

Although most Iowans can participate in the caucuses, few do.

There are 600,572 registered Democrats and 574,571 registered Republicans in the state, with an additional 737,054 registered without declaring a party allegiance. Most estimates are that somewhere north of 100,000 will show up in each party. In 2004, 124,000 Democrats took part, and about 90,000 Republicans caucused in 2000 for the last contested GOP event.