A top foreign policy adviser to Democratic presidential contender Barack Obama resigned Friday after she referred to rival Hillary Clinton as a monster.  VOA National correspondent Jim Malone has the latest on an increasingly bitter campaign for the Democratic Party's presidential nomination.

Samantha Power is a foreign policy expert and a professor at Harvard University in Masschusetts.

Power had been advising Barack Obama, but quit on Friday after a Scottish newspaper quoted her as describing Hillary Clinton as a monster.

In her resignation statement, Power expressed regret for her comment and said she had often expressed admiration for Senator Clinton in the past.

Power's comment came to light after a top aide to Senator Clinton, Howard Wolfson, compared Senator Obama to Ken Starr, the special prosecutor who carried out a lengthy and controversial investigation of the Clinton White House during the 1990s.  That comment offended some Obama supporters.

The increasingly negative tone of the campaign comes as Clinton attempts to portray Obama as too inexperienced in foreign policy and national security matters. 

"National security will be front and center in this election," she said.  "We all know that.  And I think it is imperative that each of us demonstrate that we can cross the commander in chief threshold."

Obama continues to hold a delegate lead of about 100 over Clinton, despite Clinton winning three of four primaries last Tuesday, including the large states of Ohio and Texas.

Obama told ABC News his campaign may be more aggressive in responding to the Clinton attacks.  He also said the message of his three losses on Tuesday may be that he needs to work harder in the upcoming primary and caucus contests.

"People started saying, well, maybe we want this to continue a little further," he said.  "They want me to earn this thing and not feel as if I am just sliding into it."

Wyoming holds a Democratic caucus on Saturday and Mississippi holds a primary on Tuesday.  But the next big battle looming is the Pennsylvania primary on April 22.  Clinton is favored in Pennsylvania, but a new poll shows Obama has cut into her lead there.

Meanwhile, the presumptive Republican nominee, Senator John McCain of Arizona, is focused on unifying his party and beginning the process of choosing a vice presidential running mate.

McCain campaigned in Georgia Friday and reminded his audience that his support for the military surge policy in Iraq stands in sharp contrast to the troop withdrawal plans put forward by Obama and Clinton.

"My Democratic friends said they want to set a date for withdrawal.  That is a date for surrender," he said.  "That is date where al-Qaida would announce they have defeated the United States of America."

McCain also told supporters that tax cuts and job retraining are keys to bolstering the weakening U.S. economy.  Concern over the economy now ranks as the number one issue for U.S. voters in the campaign.