Miami/Washington:  Democratic presidential front-runner Barack Obama has accused President George W. Bush of complicity in the rise of his most fervent nemesis in Latin America through what Obama called a "negligent" US foreign policy that has created a void for anti-American leaders to extend their reach in the region.

"No wonder, then, that demagogues like Hugo Chavez have stepped into this vacuum," Obama said, referring to Venezuela's president. Obama made the charge before a luncheon at a Miami hotel hosted by the Cuban-American National Foundation. Obama also delivered a bolder call for "direct diplomacy" with dictator Raul Castro, a policy that John McCain has dismissed as naive and counterproductive.

On Friday, Hillary Clinton invoked the 1968 assassination of Robert F. Kennedy in explaining her decision to remain in the race for the nomination, comments that drew criticism from aides to Obama and cooled speculation that the two may form a joint ticket for the general election.

Clinton, asked during a meeting in South Dakota about continuing to run despite long odds of winning the nomination, said that while the media and Obama's campaign have urged her to withdraw, "historically, that makes no sense".

"My husband did not wrap up the nomination in 1992 until he won the California primary somewhere in the middle of June, right?" she continued. "We all remember Bobby Kennedy was assassinated in June in California."

Clinton quickly launched a damage control effort, saying: "I regret that if my referencing that moment of trauma for our entire nation, and particularly for the Kennedy family was in any way offensive. I certainly had no intention of that,