Still, the outcomes for both parties depended on many variables, particularly in the Democratic contest, where Sens. Joseph R. Biden Jr. of Delaware and Christopher J. Dodd of Connecticut, and Gov. Bill Richardson of New Mexico staged vigorous campaigns. Candidates who do not reach a threshold of 15 percent in each precinct are deemed non-viable, freeing their supporters to align with a more-popular candidate.

The Republican contest has no such rules, but, for Democrats, the realignment of supporters could tip the balance of the contest, so the leading campaigns intensified efforts on Wednesday to identify voters' alternative selections. Those decisions also often hinge on local politics, adding a fresh dose of uncertainty to a volatile caucus night.

On the final day of campaigning, Obama's brigade worked to solidify support from independent men, moderate Republican women and younger voters — most of whom have never caucused before — to address concerns about the depth of his support in rural Iowa, which has disproportionate sway in the final outcome. He worked his way through four stops in eastern Iowa before holding a nighttime rally in Des Moines, with each event designed to be a meeting place for supporters to pick up materials for a door-to-door canvassing effort.

Clinton's team, meanwhile, dispatched 625 people to knock on the door of every voter who has signaled support and every undecided voter leaning her way. Her advisers lowered expectations for Thursday night, saying there would be no winners or losers if the three leading candidates finished closely bunched together.

After Clinton spoke in Cedar Rapids on Wednesday afternoon, her best-known supporter in Iowa, the former governor, Tom Vilsack, a co-chairman of her campaign, said he would not predict the outcome. But Vilsack also asserted that while he did not know whether Clinton would "win," he had no doubt that she had already "succeeded" in Iowa.

"When she got into this in March, she wasn't competitive here — she wasn't competitive," Vilsack said, putting it as plainly as he ever has that Democrats here were skeptical of a Clinton candidacy early on, leading a deputy campaign manager to suggest last spring that Clinton skip Iowa altogether.

Clinton was running statistically even with Obama and Edwards in some early polls, although she was quite a bit behind them in others.

Edwards waged a 36-hour marathon campaign swing, making 16 stops from the Missouri River in the west to the Mississippi River in the east. He also looked beyond Iowa by releasing an advertisement to begin running Wednesday in New Hampshire, which holds its primary on Tuesday, pledging to fight against corporate greed that has "infiltrated everything that's happening in this democracy."