DES MOINES, Iowa -- Winter finally announced itself in Iowa this weekend, a biting, swirling rain that symbolized the launch of something else: After a long windup, the race for the presidency is on.

“No one does it better than Iowa,” Mitt Romney said to hundreds of people gathered outside a Hy-Vee supermarket Friday morning. “Look at you out here. With this rain, with the cold, with the wind.”

The preliminaries are over. Tuesday night more than 100,000 hearty Hawkeye State Republicans — about a quarter of the votes cast in a typical Miami-Dade countywide election — are expected to gather at nearly 1,774 meetings across the state to winnow the field of Republican presidential candidates.

“We’re the kick-start,” said Urbandale resident Larry Mersereau, 60, who came out for Romney on Friday but, like many here, was trying to finalize his decision and plans to see Newt Gingrich on Sunday. “I wouldn’t be out here in this if I didn’t take it seriously.”

The nominating process culminates in nine months with the Republican National Convention at the Tampa Bay Times Forum, but Iowans will reduce the options for the rest of the country’s GOP voters.

Overwhelmingly dominated by evangelical activists, the caucuses tend to be better at establishing losers — fourth place or worse usually kills a campaign — than winners. Of the past six contested Republican caucuses, only victors Gerald Ford, Bob Dole and George W. Bush went on to win the nomination. John McCain finished fourth in 2008.

But the verdict in Iowa this week may determine whether the Republican primary wraps up quickly or not.

“I cannot imagine a scenario where a candidate takes both Iowa and New Hampshire and is not the nominee. At that point the momentum is so strong,’’ said Tallahassee-based Republican consultant Sally Bradshaw, who is not affiliated with any campaign.

A poll for the Des Moines Register released Saturday night showed Romney with 24 percent support, followed by Ron Paul at 22, Rick Santorum at 15, Gingrich at 12 percent, Rick Perry at 11 and Michele Bachmann at 7 percent. But the poll, which had a margin of error of four percentage points, also showed Santorum surging in the final days before the caucuses.

The Republican contest has been one of the most volatile and unsettled in memory, even as the fundamental story line remained consistent: Romney the nominal front-runner struggling to win over more than 25 percent of primary voters, while the remaining 75 percent splintered among the rest of the field.

While Romney never truly shut down his 2008 campaign, influential Republicans courted a roster of alternatives: Mitch Daniels, Haley Barbour, Chris Christie, Jeb Bush.

They all passed.

In August, Perry, the Texas governor, saw an opening and he quickly shot to the top of the polls. Prone to gaffes and poor debate performances, he crashed just as hard, as did Bachmann and Herman Cain, who dropped out after several women came forward to accuse him of improper behavior.

The field was finally set in October — later than most election cycles — when Sarah Palin announced she would not run, ending a protracted tease that included a campaign-style bus tour in Iowa during the Ames straw poll in August.

Most recently it has been Gingrich’s turn at the top and only weeks ago he was boasting about, “when I’m in Tampa accepting the nomination.” But his support has eroded under media scrutiny and millions of dollars in negative TV ads by rivals. Nearly half of all the ads shown in Iowa have been attacks on Gingrich, according to Kantar Media’s Campaign Media Analysis Group. Only 20 percent were negative toward Romney.