By tradition, the first primary held the power to propel winners into the rush of primaries that follow - to send the also-rans home for good.
And by registration, New Hampshire's balance of power rested with its independent voters, more than 40 percent of the electorate, neither reliably Democratic nor Republican, with the power to settle either race, or both.
Turnout was reported to be brisk on a spring-like day.
McCain, an Arizona senator, in particular, appealed for their support in the run-up to the primary. He battled Romney, the former governor of next-door Massachusetts, and to a lesser extent Mike Huckabee, the former Arkansas governor who won last week's Iowa caucuses.
"It has all the earmarks of a landslide with the Dixville Notch vote," an upbeat McCain quipped as his campaign bus headed to a polling place in Nashua. The crowd of supporters there was so big, that voters complained and a poll worker pleaded with McCain to leave. Seconds later, the bus pulled away.
Former New York Mayor Rudy Giuliani, former Tennessee Sen. Fred Thompson, Texas Rep. Ron Paul and California Rep. Duncan Hunter completed the Republican field.
Obama, too, hoped independent voters would come his way, as they did last week in Iowa, where he won the first test of the campaign. Clinton, the New York senator and former first lady, ran third in Iowa. Former Sen. John Edwards of North Carolina was second.
Obama drew huge crowds as he swept into New Hampshire, and as the front-runner drew plenty of attacks from Clinton and her husband. Asked if he expected more, Obama said, "Oh, I don't think it will be just in the next few days. I think it'll be, you know, until I'm the nominee or until I quit." He said he understood their frustration.
Clinton, for her part, retooled her appeal to voters on the run. She lessened her emphasis on experience, and sought instead to raise questions about Obama's ability to bring about the change he promised.
Win or lose, she said she was in the race to stay - never mind Edwards' suggestion that the voters of Iowa had told her that her presence was no longer needed.
There was no letup in the television ad wars.
TNS Media Intellligence.cmag, a firm that tracks political advertising, said Clinton spent $5.4 million to reach New Hampshire voters, and Obama spent $5 million. The total for Edwards was $1.7 million, reflecting a smaller campaign treasury. New Mexico Gov. Bill Richardson, fourth candidate in the race, could afford about $500,000.
As happened in Iowa, Romney spent more than his rivals combined on television for the New Hampshire primary.
After losing Iowa, he could ill afford another defeat after basing his campaign strategy on victories in one or both states. Reflecting the stakes, he clashed in weekend debates with Huckabee over the Iraq war and with McCain over immigration as he tried to right his campaign.
On Tuesday, Romney put a positive face forward. "The Republicans will vote for me," he said. "The independents will get behind me."
McCain, too, was in need of a victory. Once the perceived front-runner, he suffered through a near-death political experience last year when his fundraising and support collapsed. He rallied, and by the final days of the New Hampshire race, held a celebration of sorts to mark his 100th town hall meeting in the state he won eight years ago.
