In a presidential race in which uncertainty remains the order of the day, the leading Democratic candidates agree on at least one thing: no state's contest will be more important in determining the party's nominee than Iowa. Despite all the jockeying by other states to encroach upon Iowa's traditional first-in-the-nation caucuses, a win this time in Iowa is widely considered the essential prize for Democrats. That is why the candidates continue to visit the state regularly, opening up nearly 100 field offices around the state among them and spending time signing up supporters.

Much is still uncertain about Iowa's caucuses — from when exactly they will be held (on a date still to be determined, shortly after New Year's) to who is ahead in the essential task of building an organization and popular support in the state. Democrats are convinced Iowa is the key to the nomination, which has created a self-fulfilling prophecy: the more important Iowa becomes, the more the candidates visit and buy television advertising, raising the stakes even higher.

All this explains Sunday's extraordinary spectacle. In the middle of a field also used for an annual hot-air balloon festival (insert your own joke here), the six top Democratic candidates spoke to their biggest Iowa crowd of the campaign so far — organizers said it topped 14,000 people — delivering stump speeches with as much passion and energy as they could muster. It was an unusual setting, with the six candidates required to sit on the stage together and listen to each other's remarks, with open fields, farm equipment and a giant American flag serving as the backdrop.

The candidates came to the annual political fund-raising event at the invitation of Iowa's Democratic Senator Tom Harkin, who has said he does not expect to endorse a candidate in the presidential race. Harkin has been holding his steak fry for 30 years, and it has been the scene of many memorable speeches, including one from Obama last year that effectively launched his political campaign. The steak fry is among the most anticipated and highly scrutinized Iowa political events leading up to the caucuses. This year, attendees paid $30 per ticket (many provided by the presidential campaigns themselves) to eat steak and beans, endure the strong sun and hear the candidates of a party that is fired up about ending the war in Iraq and ending eight years of control of the White House by the Republicans.

The three leading candidates — Senator Hillary Clinton of New York, Senator Barack Obama of Illinois and former Senator John Edwards of North Carolina — each had hundreds of supporters on hand, creating the atmosphere of an outdoor political convention, with signs, campaign buttons and eager volunteers sporting campaign paraphernalia and chanting slogans.

The forum stressed unity over intra-party division. All the candidates expressed their disdain for Bush and a military conflict whose unpopularity fuels both Democratic anxiety about the state of the world and hope that the party will prevail in 2008; promises for universal health care and energy reform; and, above all, a powerful aspiration for change. The unusual unity of purpose and optimism, for a party used to being divided and often dispirited, was reflected in the crowd, whose fired-up members often cheered for the remarks of candidates not their own.

Obama, speaking first, directly addressed the primary question about his candidacy — whether he has enough experience to become the President in 16 months. He testified about his judgment and his character and reminded the crowd of his initial opposition to the war, a centerpiece of his candidacy. He also pledged for the first time to support future war funding bills in Congress only if they contain deadlines for bringing American troops home.

Clinton also tackled head on a major doubt about her: the prospect of a Clinton restoration leading to stalemate in Washington and a continuation of an old style of politics that many voters disdain. She repeated her campaign-trail pledge, "If you're ready for change, I'm ready to lead," and even cited the new book by former Federal Reserve Chairman Alan Greenspan, a Republican, which praises Bill Clinton's record on the economy.

Edwards, who is stronger in Iowa than he is nationally, has staked his candidacy on winning the caucuses and on a populist appeal meant to contrast with Clinton's more cautious centrist approach on health care, Iraq and the role of lobbyists. In the day's lone confrontational moment, he said that simply declaring yourself a candidate of change does not make it so. It was an apparent reference to Clinton, who sat right behind him on stage, though Obama may have felt the sting as well.

The next tier of candidates — New Mexico Governor Bill Richardson, Connecticut Senator Chris Dodd and Delaware Senator Joe Biden — spoke as well, but they did not have the same numbers of supporters on hand, hit on most of the same themes, and did not attempt to challenge the front-runners directly. That will make it harder for them to break through. And breaking through in Iowa is what it is all about for the Democratic candidates, now and for the next four months.