The winner in Iowa's kick-off caucuses can count on a boost in the marathon struggle for the Democratic presidential nomination but it's no guarantee of ultimate success.

Which helps explain why Howard Dean (search), John Kerry (search), John Edwards (search) and Dick Gephardt (search) invested many months and millions campaigning across the state.

And why Wesley Clark and Joe Lieberman didn't.

"The winnowing begins tonight," said Democratic chairman Terry McAuliffe (search), who flew into the state to watch it begin.

Alone among this year's contenders, Gephardt learned this lesson the painful way. The Missouri lawmaker won the caucuses in 1988, then quickly faded in the primaries that followed and was gone from the race before the winter's snows had melted.

Michael Dukakis (search) won the nomination that year -- he of the third-place finish in Iowa.

Dean showed he understands it as well. The former Vermont governor was the only major contender willing to leave Iowa on the campaign's final weekend. He turned up in Plains, Ga. on Sunday to stand alongside Jimmy Carter (search) -- who sort of won Iowa in 1976. "Undecided" led at evening's end that year, but the Georgian -- like Dean, a former governor -- finished first among the named candidates. It was a startling showing that launched him on the road to the White House.

Kerry and Edwards know the same thing.

A few weeks ago, before a turn of fortunes in the polls, Kerry's campaign aides were claiming that a close third-place finish would signal enough support for the Massachusetts senator to propel him forward in the race.

Edwards, who has avoided the type of political attacks his rivals have employed, said during the day he, too, could survive a loss and remain a viable candidate.

Despite campaign attempts to shape expectations, it's up to the campaign donors and political players in the primary states ahead to decide what Iowa's caucus results mean for each of the four, if anything.

A disappointing showing translates into less money, which means fewer campaign commercials, which leads to poor results in later primaries. It's a downward spiral that caught Gephardt in 1988, and it's difficult to escape.

After sitting out Iowa, Lieberman, the party's 2000 vice presidential nominee, and Clark, a retired general, get their first taste of primary combat next week in New Hampshire.

Dean leads the polls there, with Clark gaining and Lieberman lagging.

While most Democrats focused their attention on Iowa, the former general moved during the day to strengthen his hand in New Hampshire. The Clark campaign said it was in negotiations with a veteran of Republican Sen. John McCain's 2000 presidential campaign, John Weaver, to be a senior strategist.

McCain won New Hampshire's primary over Bush in 2000 with a strong appeal to independent voters, permitted to vote in either primary.

Lieberman said his fortunes would change.

"We're going to close with a big finish," he said. "Look how Iowa changed in a week."

That was a reference to a big swing in perception -- if not in fact -- that Kerry and Edwards were on the upswing in the final days of the caucus race, and that Gephardt and Dean were not.

For his part, McAuliffe sounded relieved that after months of campaigning, it was time for the voters to speak. He forecast that the party will select a nominee by March 10.

If he's right, then the main event will begin for Democrats, contesting a Republican in the White House who leads in the polls as he embarks on a quest for a second term.