reached out for the Republican delegates needed to secure his nomination after a decade's struggle.
In all there were 370 Democratic delegates at stake in Rhode Island, Vermont, Ohio and Texas, which uses an unusual primary-caucus system.
Polls close at 7 p.m. EST in Vermont, 7:30 p.m. EST in Ohio, 9 p.m. EST in Rhode Island, and at 8 p.m. EST in Texas except for El Paso and Hudspeth counties, which close at 9 p.m. EST, and the Texas Democratic caucuses begin shortly after polls close. Check CBSNews.com for results as they come in.
After 11 straight victories, Obama had the momentum and the lead in the delegate chase in CBS News count, 1,390-1,276. See the latest CBS News state-by-state delegate tally.
Clinton in desperate need of a comeback with time running out - if it hadn't already.
"Hillary Clinton, if you believe the polls, and that's always a danger, seems to have made her move in the last couple of days," CBS News senior political correspondent Jeff Greenfield said. "I think part of that may have to do with her pounding away on the fact that Barack Obama doesn't have the experience - that so-called 3:00 a.m. ad."
Some of her supporters, her husband, the former president among them, said she needed to outpoll Obama in both Texas and Ohio to sustain her candidacy.
Without conceding anything, Obama's allies said even that wouldn't be enough, given his lead in the delegate count and party rules that virtually assure primary losers a significant share of the spoils.
Nevertheless in appearances Tuesday, Clinton sounded like she might continue her campaign if she only won Ohio, and Obama sounded almost resigned to an extension of the nomination battle.
"You don't get to the White House as a Democrat without winning Ohio," Clinton said in Houston.
"My husband didn't get the nomination wrapped up until June (in 1992). That has been the tradition," she added, without mentioning that this year most primaries were held much earlier than in 1992. "This is a very close race."
In San Antonio, Obama called Clinton "a tenacious and determined candidate" and predicted little shift in his delegate lead no matter who won Texas and Ohio, "which means that either way, we'll go on through Mississippi and Wyoming next week." Pennsylvania, the biggest single prize left, follows on April 22.
"All those states coming up are going to make a difference," he said. "What we want to do is make sure we're competing in every single state."
It takes 2,025 delegates to win the Democratic nomination, and slightly more than 600 remained to be picked in the 10 states that vote after Tuesday.
The Democratic marathon was in contrast to a Republican race that was fierce while it lasted, but long since settled.
McCain, the Arizona senator, began the night with 991 delegates, out of 1,191 needed for the nomination at the party convention next summer in St. Paul, Minn. There were 256 Republican delegates at stake in the four states on the night's ballot.
McCain's sole major remaining rival, former Arkansas Gov.had 215 delegates, and posed no threat.
It was McCain's second run at the nomination, after his loss to George W. Bush in 2000. Once the front-runner, his campaign nearly imploded last summer. But he regrouped, reassuming the underdog role that he relishes, and methodically dispatched one rival after another in a string of primaries in January and early February.
In the other half of the most wide-open presidential campaign in a half-century, Obama looked for the knockout blow, while Clinton sought a revival.
As before, he outspent her in television commercials, an advantage padded by unions working in his behalf.
Rhode Island and Vermont received little attention from either of the candidates, who devoted most of their time to Ohio and Texas. They debated once in each big state, and stressed issues that varied from one to the other.
Thus, NAFTA was a focus of the Ohio race.
Obama sent out mass mailings that said Clinton had supported the free trade agreement when it was passed during her husband's administration, and that he had opposed it. She angrily accused him of distorting her record.
But roles were reversed in the campaign's final hours after a memo surfaced in which a Canadian official described a meeting in which Obama's senior economic adviser said the Illinois senator's criticisms of the trade agreement were political positioning.
Clinton said Obama had given a "wink-wink" to Canada on the issue.
Obama said, "Nobody reached out to the Canadians to try to assure them of anything."
The Texas campaign revolved more around readiness to serve as commander in chief.
Clinton aired a television commercial that showed children asleep in their beds. "It's 3 a.m. and your children are safely asleep. Who do you want answering the phone?" the announcer said.
Obama wasn't mentioned, but responded quickly.
He told reporters that Clinton had already had her "red phone moment," - and voted for the Iraq war.
He launched his own ad, with sleeping children and a telephone ringing ominously.
"In a dangerous world, it's judgment that matters," the announcer said.