The rivals for the Democratic presidential nomination are taking part in a television debate in Texas.
Democratic presidential candidate Hillary Clinton has accused rival Barack Obama of political plagiarism during a live televised debate.
Next month's primary in the state is seen as crucial for both Hillary Clinton and Barack Obama.
But Mr Obama brushed off the accusation that he represented "change you can Xerox", saying it was just part of "silly season" politics.
Mr Obama is now the favourite for the party's nomination, and he gained even more momentum by winning the Democrats Abroad primary on Thursday.
The debate took place in Texas, which holds a crucial primary nexth month.
The Texas debate is a major test for the flagging Clinton campaign, coming after 11 consecutive poll defeats.
It comes in the wake of Mr Obama's victory among Democrats living abroad - his 11th straight win over Mrs Clinton.
Along with Ohio, Texas votes on 4 March, in a contest that is widely being seen as a must-win for the former first lady.
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.
She has not won a nominating contest in more than two weeks.
Call for change
Call for change
Mrs Clinton opened Thursday night's debate at the University of Texas in Austin by saying it was up to those voting in forthcoming state primaries to decide the outcome.
"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
Supporters of both contenders rallied ahead of the debate
"If you'll be part of this campaign - which is really your campaign, about your futures, your families, your jobs and your health care - we'll continue to make a difference for America," said Mrs Clinton.
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.
Mr Obama, for his part, reiterated his call for change.
But Mr Obama was disdainful in his response.
"Washington has become a place where good ideas go to die, because too many politicians are interested in scoring points rather than bridging their differences to get things done," said the Illinois senator.
"What we shouldn't be doing is tearing each other down, we should be lifting the country up," he said.
The Texas debate is the first of two to be held ahead of 4 March primaries in Texas, Ohio, Rhode Island, and Vermont.
On foreign policy, Mrs Clinton criticised Mr Obama for implying that he would meet the leaders of Cuba and Iran "without preconditions".
In previous televised debates, Mrs Clinton has attempted to outweigh Mr Obama's rhetoric with her grasp of policy issues.
Mr Obama countered that the new US president should reach out to both countries, but conceded that preparation for such meetings was vital.
But Mr Obama has brushed off criticism that he lacks substance.
The two senators also differed over the economy and how to extend health care coverage.
The Texas debate was the first of two to be held ahead of the 4 March primaries.
Union backing
Union backing
Mrs Clinton now needs a majority of the remaining delegates to beat Mr Obama.
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.
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.
Mrs Clinton has 1,264 delegates.
Correspondents say the blue-collar vote will be crucial in the Ohio and Texas contests, and the Clinton campaign has already begun targeting lower-income workers in its ads.
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.
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.
Are you in next month's key primary states of Ohio or Texas? How do you intend to vote? How crucial do you think those primaries are in this election race? Send us your comments using the form below.
Are you in next month's key primary states of Ohio or Texas? How do you intend to vote? How crucial do you think those primaries are in this election race? Send us your comments using the form below.
