Democratic presidential candidate Hillary Clinton has accused rival Barack Obama of political plagiarism during a live televised debate.
But Mr Obama brushed off the accusation that he represented "change you can Xerox", saying it was just part of "silly season" politics.
The debate took place in Texas, which holds a crucial primary next month.
It comes in the wake of Mr Obama's victory among Democrats living abroad - his 11th straight win over Mrs Clinton.
Both the Texas and the Ohio primaries on 4 March are being seen as must-wins for Mrs Clinton, and Thursday night's debate at the University of Texas in Austin was a major test for her flagging campaign.
Call for change
"If your candidacy is going to be about words, then it should be your own words," Mrs Clinton said.
Supporters of both contenders rallied ahead of the debate
She was referring to Mr Obama's apparent copying of rhetoric used by one of his supporters, Massachusetts Governor Deval Patrick, during his campaign speeches.
But Mr Obama was disdainful in his response.
"What we shouldn't be doing is tearing each other down, we should be lifting the country up," he said.
On foreign policy, Mrs Clinton criticised Mr Obama for implying that he would meet the leaders of Cuba and Iran "without preconditions".
Mr Obama countered that the new US president should reach out to both countries, but conceded that preparation for such meetings was vital.
The two senators also differed over the economy and how to extend health care coverage.
Neither candidate scored a decisive blow during the 90-minute encounter, says the BBC's Justin Webb in Washington.
But as Mr Obama, the front-runner, did not slip up, this probably means that he won, our correspondent adds.
The Texas debate was the first of two to be held ahead of the 4 March primaries.
Mrs Clinton now needs a majority of the remaining delegates to beat Mr Obama.
He has at least 1,353 - according to an Associated Press projection - of the 2,025 delegates he needs to secure the Democratic nomination at the party's convention in August.
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.
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