The speaker of the US House of Representatives, Nancy Pelosi, has called for a swift conclusion to the contest to choose the Democratic Party's candidate for November's presidential election.
Senator Hillary Clinton is trailing rival Senator Barack Obama in the number of delegates needed to obtain the nomination, and Mrs Clinton's campaign is reported to be in financial trouble.
She has rejected calls to step aside in the Democratic race. Mrs Pelosi said it was important for Democrats to be able to get behind one candidate if they expected to win the election.
Neither candidate is likely to have the 2,024 delegates needed to win the nomination after the contests end in early June.
This outcome would leave the decision up to nearly 800 superdelegates - elected officials and party insiders who are free to back any candidate.
Mr Obama meanwhile has taken aim at potential White House opponent John McCain on Iraq, saying the Republican senator could not offer a clear definition of success in the conflict and might leave US combat troops there for decades.
Speaking in Pennsylvania, which is the site of the next contest in the hard-fought Democratic race on 22 April, Mr Obama accused Mr McCain of clinging to the policies of the past.
'The problem that we've had, both with John McCain and George Bush, is there's no clear definition of success. There never was. And that's why this has been such a profound strategic error,' he said.
Also in Pennsylvania, Mrs Clinton accused McCain of lacking a clear plan to end turmoil in America's housing market.
'Senator McCain recently gave a speech on the economy, and best I could determine his plan was not to have a plan,' she said.
