US presidential hopeful Hillary Clinton has made her fiercest denunciation so far of Barack Obama, her rival for the Democratic Party nomination.
Mrs Clinton accused the Illinois senator of producing a misleading leaflet on her health care policy.
"Shame on you, Barack Obama!" the New York senator said at a rally in Ohio, which holds its primary in 10 days.
But Mr Obama said he stood by the leaflet, saying he was puzzled by what he called his rival's change in tone.
"Enough with the speeches and the big rallies and then using tactics that are right out of Karl Rove's playbook," said the former first lady ahead of Ohio's crucial primary early next month.
Both the Ohio and the Texas primaries, both being held on 4 March, are being seen as must-wins for Mrs Clinton.
Mr Obama, who has won 11 consecutive primaries and caucuses in recent weeks, is now seen as the Democratic front-runner.
But Mrs Clinton's campaign has struggled to find an effective way to cope with her rival's extraordinary momentum and has decided to "go negative", says the BBC's Kevin Connolly in Washington.
She and her advisers have clearly calculated that the state of the race now calls for sharper elbows and a sharper tone, our correspondent adds.
Mr Obama now has at least 1,353 of the 2,025 delegates he needs to secure the Democratic nomination at the party's convention in August, according to an Associated Press projection.
Barack Obama has pulled ahead of Hillary Clinton in recent primaries
Mrs Clinton has 1,264 delegates. Texas and Ohio have a combined total of 334 delegates up for grabs.
Correspondents say the blue-collar vote will be crucial in both contests, and the Clinton campaign has already begun targeting lower-income workers in its ads.
But in his drive to become the first black US president, Mr Obama has recently gained support from some powerful unions, including the Teamsters and the Service Employees International Union.
Meanwhile, John McCain was given a further boost by the Pacific islands of Northern Marianas which chose its nine Republican delegates on Saturday.
The islands are among three US Pacific territories each sending nine delegates to the Republican convention in Minnesota this September, and delegates have praised the former Vietnam prisoner for his knowledge of their islands.
Republicans in American Samoa also announced that all nine of their delegates would support Mr McCain.
Guam Republicans take their decision on 8 March.
The latest results give the Arizona senator a total of 976 delegates, according to the Associated Press, and he needs 1,191 delegates to secure the Republican nomination.
His rival, former Arkansas Governor Mike Huckabee, trails far behind with 254 delegates.
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