Campaign staffers for Democratic presidential hopeful Barack Obama say he will soon launch an assault on the ethics and transparency of his opponent, Hillary Clinton.
The Clinton campaign responded Sunday that Obama is going negative because momentum has turned against him in their contest for the Democratic Party's presidential nomination.
Obama holds a narrow lead over Clinton in party delegates, who will ultimately decide who wins the Democratic Party race.
With such a close contest, neither Obama nor Clinton has nearly enough pledges of support to guarantee victory at the party's convention later this year. Still, one of the leading U.S. Democrats, House of Representatives Speaker Nancy Pelosi, says she believes the party will agree on a nominee long before the party meeting in August.
Pelosi said Sunday that she expects either Obama or Clinton will emerge as the clear frontrunner in the coming weeks. She added that she does not expect the two competitors to join forces and run on a joint ticket - for president and vice president.
Pelosi told an interviewer on ABC Television's This Week program that her forecast is based on her lengthy experience in politics.
In the Republican Party, Senator John McCain of Arizona already has won the support of enough delegates to clinch the presidential nomination.
The Democrats' nominating system is different than the Republicans. High-ranking party officials and Democratic officeholders who hold so-called "superdelegate" status are not obliged to support the winning candidates in their states' primary elections or caucuses, and can vote freely at the party convention.
About 20 percent of all Democratic Party delegates are "superdelegates." Since the race between Senators Clinton and Obama is so close at this point in the campaign, it appears increasingly likely that a majority of the superdelegates will decide who gains the party nomination.
Some information for this report was provided by AP and Reuters.