The voting came a day after both Obama and Clinton made personal appeals here, and after Illinois Sen. Obama won contests in Louisiana, Nebraska, Washington state and the Virgin Islands.
Organizers expected heavy participation at the caucuses, but up to 8 inches of snow was forecast, along with Arctic cold, when many of the gatherings were scheduled. Even so, the Democrats started the day with more than 4,000 absentee ballots in hand.
Arden Manning, the party's executive director, said the weather wouldn't hurt turnout.
"We live in Maine, we deal with snow all time," he said. "I can't imagine snow's going to keep people at home and keep them from having their voices heard."
Though Maine's delegate count is small, Clinton and Obama, along with surrogates, came to the state as their campaigns drew tighter after Super Tuesday.
Thousands of people packed the Bangor Auditorium to hear Obama on Saturday and hundreds more who weren't allowed inside greeted him as he arrived. People also were stopped at the door as Clinton held a town hall-style gathering about 10 miles away at the University of Maine at Orono. She later campaigned in Lewiston.
Clinton's daughter, Chelsea, and husband, Bill, also visited while Obama supporter Sen. Edward M. Kennedy of Massachusetts stumped in two Maine cities in the days before the vote.
Both campaigns hit Maine heavily with radio and TV advertising, and voters' homes were being called with pre-taped messages in support of both candidates.
On Sunday, Massachusetts Gov. Deval Patrick, Rep. Patrick Kennedy of Rhode Island and Sen. John Kerry of Massachusetts, the 2004 Democratic presidential candidate, were scheduled to visit Maine caucuses on Obama's behalf.
On Clinton's side, Maine Gov. John Baldacci, Massachusetts Rep. Jim McGovern and New York Rep. Gregory Meeks were to campaign.
The high level of excitement across the state contrasted with earlier expectations that the post-Super Tuesday timing of the caucuses would dampen voter interest.
A competitive GOP race a week earlier also helped enliven interest in the Maine Republicans nonbinding caucuses, which were won by Mitt Romney. He dropped out of the race last week, making it likely that Arizona Sen. John McCain would become the GOP nominee.