(CBS/AP) Barack Obama sought a knockout against rival Hillary Rodham Clinton as voting began Tuesday in crucial contests in Texas and Ohio, where the former first lady desperately needs a win to salvage her once-powerful candidacy for the Democratic presidential nomination.
Both expressed confidence in their chances the previous night, which they spent in the same Texas city. But both teams also acknowledged that split decisions and close votes could prolong the battle for at least another month - or more.
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.
"We know this has been an extraordinary election. It continues to be. We're working hard to do as well as we can," said Obama, who planned to await Texas returns in San Antonio.
"I'm just getting warmed up," Clinton told reporters, a clear sign that she expects to press the campaign on beyond Tuesday no matter the outcome.
"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."
Clinton planned to open Election Day in Houston and Dallas, but then head back to Ohio for more campaign events. She will await results in Columbus before returning to Washington Tuesday night.
Freezing rain was forecasted for northern Ohio, and the southern part of the state was under threat of heavy rain and flooding.
"I've never actually seen a campaign where weather has been a factor," Greenfield said. "We talk about it all the time, but one of the questions is if the weather's this lousy, who doesn't turn out? The young kids supposed to go for Obama? First-time voters? Older people, who are Clinton's strongest age group?"
Obama began the day with a stop at a livestock show in Houston, where he shook hands with people at a Future Farmers of America exhibit and viewed show cows and bulls.
Polls show tight races in both Texas and Ohio. The Obama campaign saw Texas as their best opportunity, while the Clinton campaign saw Ohio as theirs. Texas offers 228 delegates, Ohio 164.
But, says CBSNews.com senior political editor Vaughn Ververs, "even if Clinton does well enough to argue for hanging on through Pennsylvania on April 22, she may face more pressure to reconsider doing so." (Read more)
"Seven more weeks of increased back-and-forth could sap the momentum of even this energized Democratic race," Ververs said.
Voters also went to the polls Tuesday in Rhode Island, which offers 21 delegates, and Vermont, 15.
"Your voice can win an election," Obama told a noisy late-night rally in Houston. Repeating a signature election refrain, he shouted: "I have only one question for you: Are you fired up? Ready to go?" When the crowd roared its approval, he added: "Let's go change the world."
His wife, Michelle, had a more subdued message for the cheering supporters: "We have a lot of work to do."
Eric Gingerich, 36, a junior high social studies teacher, normally votes Republican, but voted Tuesday for Obama at an elementary school in Hilliard, Ohio, near Columbus.
"I like how he can bring the two parties together and the country together," he said.
Republican scandals in Ohio have made him more open to Democrats, but Gingerich said Clinton is too polarizing.
"If anybody is going to pull me over to the Democrats it's Barack, not Hillary," he said.
The economy was on the mind of Gretchen Genung, 61, a state employee who voted for Clinton in Cincinnati.
"We know where the economy is right now, we know where it's going and we need somebody in there like Hillary who has the experience," Genung said.
Obama spent Monday campaigning in Texas, emphasizing his readiness to take over as commander in chief.
(AP Photo/Nam Y. Huh)But he was dogged by allegations that he had overstated his opposition to the North American Free Trade Agreement to win votes in Ohio; and his ties to Chicago businessman Antoin "Tony" Rezko on the day that jury selection began in the political corruption trial of the real estate developer and fast-food magnate.
"Tony Rezko was a friend and supporter of mine for many years. These charges are completely unrelated to me, and nobody disputes that," Obama said at a news conference in San Antonio.
Obama did receive a $10,000 contribution made by a Rezko associate that is mentioned in the indictment. But Obama's campaign has long since sent the money to charity.
"There's no dispute that he raised money for us, and there's no dispute that we've tried to get rid of it," Obama said.
Meanwhile, Clinton waged yet another marathon day on Monday, shaking hands in the chill pre-dawn darkness with workers at a Chrysler factory in Toledo and winding up in Texas. She predicted a strong showing in the primaries and said she was looking ahead to contests down the road like Pennsylvania on April 22.
She sounded a populist economic theme as she courted voters who have suffered with the decline of manufacturing in the industrial Midwest and Ohio. Then, in military-friendly Texas, Clinton broadened her theme to include veterans' issues and to trumpet her backing from a string of top military officers.
Clinton worked to underscore her core campaign theme that she's the more experienced on the issue. She held a one-hour town hall meeting where she strode the stage surrounded by a friendly audience and took questions selected from the thousands that were submitted on issues ranging from health care to education to veterans issues. Her campaign purchased time on a sports-oriented cable network to broadcast the event around the state, and the event was streamed on the campaign's Web site.
Both Clinton and Obama launched new television spots in Texas and Ohio to make their closing arguments.
Clinton ended her day with a rambunctious rally in Austin before flying to Houston.
Obama campaign manager David Plouffe called Tuesday "the last big window of opportunity" for Clinton, noting that "enormous leads" she enjoyed as recently as two weeks ago had dwindled or evaporated.
Still, he said Obama was mindful that "this could go on for some time. We're prepared for whatever situation occurs."
Republican presidential contenders John McCain
and Mike Huckabee also campaigned in Texas, though voter interest centered on the closer Democratic race.