Obama said a ''split decision'' was likely. ''I don't think today's going to end up being decisive,'' he said.
A frigid rain fell in parts of the northeast, part of a wintry mess expected in the region, and voters braced for snow in a large corridor from southwest Kansas to northern Michigan. A winter onslaught was forecast in Illinois later in the day.
In Topsfield, Mass., where a steady stream of voters filed to a polling place in a cold rain, teacher Marcia Spector, 58, made the ''very, very tough'' decision to support Obama, reasoning he would be more able than Clinton to win the presidency in the fall. ''I just feel that he is dynamic and he is for change,'' she said. ''He doesn't bring the baggage. I think he's more electable, actually.''
It was tough, too, for Mary Jordan, 43, a teacher's aide — so tough she didn't make up her mind until she was in the polling booth. Voting Republican, she went for Romney, the state's former governor, because of his business experience, while offering no one a glowing endorsement. ''I think he's the least unlikable,'' she said. ''I really didn't like any of them.''
Romney sought until the end to exploit the right's mistrust of McCain, an Arizona senator who opposed President Bush's tax cuts when they were introduced, departed from orthodoxy on immigration, favors mandates to slow global warming and led campaign finance reforms that activists say trampled on their speech rights.
He told supporters at West Virginia's Republican nominating convention Tuesday that McCain's support for global warming curbs ''would effectively kill coal,'' a lifeblood of the state.
''This is not a long shot,'' he said of his candidacy. ''I am the candidate who can stop John McCain.''
The days of retail politicking in rustic diners was a distant memory, although just weeks old. Sens. Clinton and Obama each poured more than $1 million a day into TV ads in the last week alone; Clinton buying an hour on the Hallmark Channel for a town hall meeting on Monday night, Obama seeing some $250,000 disappear in 30 seconds in his Super Bowl ad a day earlier.
Not only was the electoral territory was vast, but so were the stakes. Romney boomeranged across the country and back in a 37-hour dash, branding himself the true Ronald Reagan conservative at every stop.
Romney campaigned from Tennessee to California on the eve of the voting, then turned back to attend the West Virginia convention.
Former Arkansas Gov. Mike Huckabee focused on the South, his continued candidacy an open question — as was Romney's viability if he couldn't pull off surprises Tuesday.
Clinton voted near her home in Chappaqua, N.Y., accompanied by husband Bill and daughter Chelsea. ''You're a Democrat, right?'' election worker Evan Norris joked. Clinton smiled. ''I am just very excited about today,'' she said. ''The stakes are huge.''
In Illinois, Obama supporters expressed pride for the home-state senator as they voted. ''We have something great to vote for today,'' said Catherine Braendel, 44, a marketing consultant who lives down the street from Obama in Chicago.
In Grayslake, Ill., registered Democrat Steve Greenberg, 39, decided his vote would be more valuable on the Republican side as he thought ahead to the general election. ''I went with McCain because if the Democrats lost, I'd be more comfortable with him.''
McCain struggled to close the sale with his party's base after coming strikingly far without its solid support. ''I will preserve my proud conservative Republican credentials'' while extending a practiced hand to Democrats, he promised.
After months when it was all about expectations and momentum, not to mention confusion, real numbers finally became important.
The two dozen contests Tuesday were delivering 1,023 Republican and 1,681 Democratic delegates. The number needed to win the nomination: 1,191 Republican and 2,025 Democratic.
John Edwards' departure after South Carolina's primary simplified the math but little else on the Democratic side.
Since winning that state, Obama has collected a succession of marquee endorsements — several of them named Kennedy — and pulled into a statistical tie with Clinton in a national poll and in California, Tuesday's biggest prize with 370 Democratic delegates.
The two were campaigning for history as well — Clinton seeking to become the first female president, Obama the first black one.
Little separates them on most issues, including universal health coverage, ending U.S. military involvement in Iraq and raising taxes on the rich. And neither has accounted fully for all their proposed spending.
Instead, the campaign has turned on her experience and his vision of change, debated intensely but with more civility in the latest round than when former President Clinton, purposefully or not, brought racial sensitivities to the surface in stumping for his wife.
Party rules were stacked against a Tuesday knockout for Democrats. All their primaries and caucuses were awarding delegates proportionately, so coming in second counted. In the Republican field, nine contests offered all delegates to the winner.
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Associated Press writers Dave Carpenter, Ashley M. Heher Jim Fitzgerald, Beth Fouhy, Glen Johnson, Jim Kuhnhenn, Nedra Pickler, Libby Quaid and Liz Sidoti contributed to this report.