The two main Democratic presidential candidates, Hillary Clinton and Barack Obama, have accused each other of negative campaigning in a TV debate.
The two White House hopefuls also attacked each other's policies on health care, trade and foreign policy, including the Iraq war.
It was their final face-to-face encounter before next week's crucial primaries in Ohio and Texas.
Mr Obama has won the previous 11 primaries and caucuses.
Analysts say Mrs Clinton needs to win a majority of delegates in both states to stay in the race to choose the Democratic candidate at the national nominating convention in August, ahead of the November elections.
Accusations of dirty tricks and negative campaigning have dogged the past week of the nomination race.
In the opening minutes of the televised debate in Cleveland, Ohio, the two politely but firmly accused each other of spreading misinformation about their policies.
"The charges that Senator Obama's campaign has made regarding fliers and mailers and other information that he has been putting out about my health care plan and my position on Nafta (the North America Free Trade Agreement) have been very disturbing to me," Mrs Clinton said.
Mr Obama retorted that his rival's campaign had "constantly sent out negative attacks on us... We haven't whined about it because I understand that's the nature of these campaigns."
Both insisted they stood for universal healthcare, and that the other's programme would leave some people without cover.
Mr Obama sought to draw a line, however, under the appearance of a controversial photograph of him wearing traditional Somali robes during a visit to Kenya in 2006.
He said he believed Mrs Clinton when she said she did not know where the photo had come from.
Mrs Clinton and Mr Obama went on to say that they would both seek to renegotiate Nafta with Canada and Mexico, under threat of opting out of the 14-year-old pact.
Many workers in Ohio and other states hard-hit by factory closures blame the deal for job losses.
It was on foreign policy that there was the strongest contrast between the two candidates, says the BBC's Jonathan Beale in Washington.
Mrs Clinton repeated her stance that her qualifications and experience as former first lady and New York senator put her in the best position to be the next commander-in-chief.
Mr Obama replied that experience should not be confused with longevity in Washington, saying Mrs Clinton's vote in favour of authorising the war in Iraq in 2002 was a massive strategic blunder.
It was hard to define an outright winner of the debate, says our correspondent, but there was nothing to suggest Mrs Clinton did enough to turn the tide that has been moving in Mr Obama's favour.
Mrs Clinton now needs to win a majority of the delegates in the remaining state primaries and caucuses to stay in the race to choose the Democratic candidate for November's presidential election.
Ohio and Texas, both big states, are being seen as must wins for her.
Several polls suggest Mr Obama is gaining ground in both Texas and Ohio. The Illinois senator is leading in Texas for the first time, with 50% compared to 46% for Mrs Clinton, according to a CNN poll.
On Tuesday, Mr Obama won endorsement from a former rival for the Democratic nomination, Senator Christopher Dodd.
On the Republican side, front-runner John McCain has also been campaigning in Ohio, as the party prepares to hold its own primary in the state, also on 4 March.