"I'm just getting warmed up," said Clinton, looking beyond this week's contests and shrugging off 11 straight primary and caucus defeats as well as a three-digit deficit in delegates.
The former first lady campaigned from Ohio, where she accused Obama of double talk on NAFTA, to Texas, where her new television commercial questioned his readiness to serve as commander in chief.
Obama spent his day in Texas, a state rich in military bases, where he pledged to begin the withdrawal of U.S. troops from Iraq next year and envisioned a "seamless transition from active duty to civilian life" for men and women who leave the armed forces.
But he was shadowed by allegations that he had overstated his opposition to the North American Free Trade Agreement to win votes back in Ohio. He told reporters his campaign never gave Canada back-channel assurances that his criticism of NAFTA, which is wildly unpopular in Ohio, amounted to political posturing.
"Nobody reached out to the Canadians to try to assure them of anything," he said at a news conference in Carrollton, Texas.
In addition to Texas and Ohio, Rhode Island and Vermont hold primaries on Tuesday. The four races have a total of 370 national convention delegates at stake.
Obama has won 11 straight contests, and leads in the Associated Press delegate count, 1,386-1,276. His margin is greater among delegates chosen in the primaries and caucuses, 1,187-1,035, while Clinton leads among party officials, known as superdelegates, 241-199.
Obama has been gaining ground among superdelegates in recent weeks as his victories have piled up, and Clinton's support has begun to erode.
At the same time, senior Democrats have begun to speak out in private as well as public about the impact a continuation of the bruising campaign might have in a fall confrontation with Sen. John McCain, the Republican nominee-in-waiting.
Several Democrats said the party's chairman, Howard Dean, told House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid last week he was concerned about the possible impact of a nominating campaign that stretched through the end of the primaries in early June. Dean also said that if the party is divided going into next summer's convention, it would remain that way afterward, even if the differences were papered over in the four days in Denver, these officials said.
Dean did not suggest any attempt to intervene. The Democrats who described his comments did so on condition of anonymity, saying they had been made in a private setting.
Dean, Reid and Pelosi, all superdelegates, are neutral in the race between Clinton and Obama.
Another superdelegate, New Mexico Gov. Bill Richardson, who dropped out of the presidential race weeks ago, said over the weekend that the candidate with "the most delegates after Tuesday, a clear lead, should be in my judgment the nominee."
Sen. Sheldon Whitehouse of Rhode Island, told Providence radio station WPRO during the day, "We can't go all the way through to the convention fighting with each other while McCain and the Republicans lob in whatever free shots they want." Whitehouse, a superdelegate who supports Clinton, added, "Let's see how Tuesday plays out, and then let's start thinking about how we're going to get behind a candidate."
Slightly more than 600 delegates will remain to be picked in primaries and caucuses in 10 states after this week's primaries, beginning with Wyoming caucuses this weekend and a Mississippi primary on March 11. Pennsylvania is the biggest state remaining, with 158 delegates in a primary on April 22.
The controversy over NAFTA flared after the AP reported the existence of a memo, written by a Canadian official, asserting that Obama's senior economic adviser had told him the Illinois senator's public criticism of the free trade agreement was "political positioning."
The adviser, Austan Goolsbee, said his comments were misinterpreted by the memo's author, Joseph DeMora, who works for the Canadian consulate in Chicago and attended the meeting.
Clinton campaigned from the pre-dawn hours until after dark as she made her way from Ohio to Texas in hopes of a political revival.
Her campaign released a new television commercial designed to undercut Obama's claim that he is ready to become commander in chief.
"Barack Obama says he has the judgment to be president. But as chairman of an oversight committee charged with the force fighting al-Qaida in Afghanistan, he was too busy running for president to hold even one hearing," it says.
The announcer adds: "Hillary Clinton will never be too busy to defend our national security, bringing our troops home from Iraq and pursuing al-Qaida in Afghanistan."
Obama aired a two-minute commercial in Ohio and Texas - the same one he used before the Jan. 3 Iowa caucuses that kicked off the election season - in hopes of nailing down at least one big-state victory.
"This country is ready for a leader who will bring us together.
That's the only way we're going to win this election," he says in the ad. "And that's actually how we'll fix health care and make college affordable, become energy independent and end this war."
