DES MOINES, Iowa - With the new year at hand, the long struggle for the presidency is about to pass into the hands of the voters, at least a few of them.

On Thursday night, the process will begin with the Iowa caucuses - 1,784 separate gatherings in which 250,000 people, more or less, will begin to trim the field of candidates from the many to a few.

Rarely has a campaign come to the eve of the voting steeped in so much uncertainty about who might prevail in either major party.

Elements in the mystery include the turmoil in Pakistan - and whether it enhances the prospects of the candidates with foreign-policy credentials - and a new political calendar in which 60 percent of all convention delegates are to be allocated by Feb. 5.

Here in Iowa, the Democrats are engaged in a tight, three-way contest involving Sen. Hillary Rodham Clinton of New York, Sen. Barack Obama of Illinois, and former Sen. John Edwards of North Carolina.

A win for Clinton would help her reestablish the sense of inevitability that has been seeping out of her candidacy ever since the debate at Drexel University two months ago. But a victory for Obama would have the opposite effect, perhaps signaling a long race ahead.

On the Republican side, the two former governors, Mike Huckabee of Arkansas and Mitt Romney of Massachusetts, are the main contenders, with everyone else battling for third place - and hoping that third will matter.

There are questions about Huckabee's ability to translate an Iowa win, which he says would be a "seismic event," into victories in states where evangelical Christians are less numerous.

But there's little doubt that Romney would be hurt by falling short; he has built his candidacy around a series of early-state successes.

Six Democratic and four Republican candidates were traversing the icy Iowa landscape yesterday, rallying supporters and hoping to find and sway whoever might still be undecided.

Even with only a few days left before caucus night, no town in Iowa was too small to merit a candidate visit.

Former Sen. Fred Thompson of Tennessee, one of the Republicans hoping to finish third, headed for Williamsburg, population 2,622. Sen. Chris Dodd of Connecticut visited Logan, which has 1,545 residents, while another Democrat, Sen. Joseph R. Biden Jr. of Delaware, was hitting Elkader, home to 1,465.

Last night, after making three stops in slightly larger communities in eastern Iowa, Edwards brought his "America Rising" tour back to Des Moines for an event that captured the political intensity that's building in Iowa as caucus night approaches.

In a high school near the state capitol, a fervent, overflow crowd of 1,000 turned out to hear the candidate promise not to hire lobbyists for corporations or foreign government for his White House - and to talk about what he's feeling as he traverses the state.

"You know, there's so much excitement and energy in this campaign," he told the cheering throng. "I can feel it everywhere I go. I'm not imagining it. It's real."

Republican candidates generally have been drawing smaller crowds here than the Democrats, a phenomenon likely to be reflected in the turnout on caucus night.

Polls indicate that Democratic voters are enthusiastic about what they consider a strong field of candidates while some Republicans are disheartened by their options.

The caucus races in the two parties differ, too, in their stakes, and that will be reflected in the time the candidates spend here between now and Thursday.

Months ago, the Democratic candidates decided that Iowa would be their main event. All of them, except for Rep. Dennis Kucinich of Ohio and former Sen. Mike Gravel of Alaska, have given it everything they have, particularly in the closing days.

Clinton arrived in the state the day after Christmas with no intention of leaving until caucus night. Her tour has the title of "Big Challenges, Real Solutions - Time to Pick a President," words chosen to stress that "she has the experience to make change happen starting on day one," as her campaign puts it.

Obama has been following a similar itinerary, drawing sharp contrasts between himself and his rivals.

"In the end, the argument we are having between the candidates is not just about the meaning of change," he tells his audiences. "It's about the meaning of hope."

Biden, Dodd and Gov. Bill Richardson of New Mexico have been here since Christmas as well. All of them will be hard-pressed to continue much past the New Hampshire primary Jan. 8 without a top-three finish in the caucuses.

Among the Republicans, only Huckabee and Romney have made similar investments of time and money in Iowa. The others appear to be hoping that staying away will insulate them against the impact of not doing well here.

Former New York Mayor Rudy Giuliani was in Iowa yesterday, his only full day here in weeks and likely to be his last before the caucuses.

John McCain, who spent parts of three days here last week, spent yesterday in his political home base of New Hampshire, where the Arizona senator hopes to win the nation's first primary.

Rep. Ron Paul of Texas, who has demonstrated considerable fund-raising prowess in recent months, was here one day last week and is to return Wednesday.

Even Huckabee, who is counting on an Iowa win, spent some of the days after Christmas in Florida, raising money in a state that will hold its primary Jan. 29, the week before the 22-state, quasi-national primary Feb. 5.

In the last four contested nomination fights, dating back to Bob Dole in 1996, the winner of Iowa has won his party's nomination. Then again, Ronald Reagan won the presidency without winning Iowa, as did George H.W. Bush and Bill Clinton.

And in one key respect, this election is unlike any in the modern era. It is the first since 1952 without a sitting president or vice president in the race.

Combine that with the accelerated political calendar, and it's hard to know what rules apply, if any. The one constant is that the Iowa caucuses are in the lead position, and here they come.

How the Iowa Caucuses Work

This is how Iowa Democrats vote for president: They go stand in corners.

Unlike a primary, which involves a quick visit to a voting booth to cast a secret ballot, the Iowa caucuses - at least on the Democratic side - are public. They can last several hours as neighbors argue among themselves, and involve a fair amount of math.

Starting at 7 p.m. Iowa time Thursday, Democrats in each of the state's 1,784 precincts will gather into "preference groups" for each candidate. Then it's time to determine which candidates meet the "viability threshold."

To remain, party rules say, a presidential candidate must have the support of at least 15 percent of caucus attendees in precincts that elect four or more delegates to the county convention; the percentage is higher in less populous precincts.

Supporters of candidates who aren't viable can merge with another candidate's group, try to recruit others to join them, form an uncommitted bloc, or just go home.

Republicans, who will also caucus Thursday, face a much simpler process: They write their selection on a slip of paper.

It will be months before Iowa's delegations to both parties' conventions are officially determined, but the results on caucus night will loom large in perceptions of the candidates' strength and thus help decide the presidential race.

- Thomas Fitzgerald

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INSIDE

Big names will join Michael Bloomberg to urge unity -

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Contact senior writer Larry Eichel at 215-854-2415 or leichel@phillynews.com.