Scolding Clinton, he said Democrats should not be "tearing each other down" but rather "lifting the country up."
New Washington Post-ABC News opinion polls showed the two hopefuls about even in Texas, with Clinton at 48 percent and Obama at 47 percent, but Clinton leading Obama in Ohio by 50 percent to 43 percent.
Obama has a growing lead in pledged delegates who will choose the Democratic candidate at the party's convention in August.
The latest count by MSNBC gives Obama 1,168 delegates to Clinton's 1,018. The Democratic nominee will likely face Sen. John McCain of Arizona, the Republican front-runner, in the election to succeed President George W. Bush.
ACCOMPLISHMENTS
Clinton also tried to raise questions about her rival's resume, pointing to a supporter's inability to list a single Obama accomplishment when pressed to do so on Tuesday in an MSNBC interview.
"So I know there are comparisons and contrasts to be drawn between us," she said.
Obama defended his record testily, saying Clinton was trying to suggest his supporters "were being duped" by him. He said he had helped push the toughest ethics reform legislation in the Senate since the Watergate scandal of the early 1970s.
"Senator Clinton has a fine record. So do I," he said.
As she has done often in recent days, Clinton questioned whether Obama was ready to take the reins of power at a time of global unrest.
Obama pointed to his opposition to the Iraq war, and Clinton's 2003 vote in favour of a Senate resolution that authorized the Iraq war, as an example of his judgment.
He said that on "the single most important foreign policy decision" of the decade, "I believe I showed the judgment of a commander in chief and I believe Senator Clinton was wrong in her judgment on that."
Cuba, now that ailing Fidel Castro has stepped down, featured prominently in the debate as the two candidates courted Hispanics who could play an influential role in the Texas vote.
Obama expressed a willingness to move quickly toward a meeting with Castro's replacement, in line with his previous commitment to hold direct talks with leaders of hostile countries if he is elected president.
Clinton was more cautious, saying Cuba should first make progress on long-standing U.S. complaints about the need to improve human rights and release political prisoners.
Castro is expected to be replaced in power by his brother, Raul Castro, 76.
