( dpa ) - The race between Democratic presidential rivals Barack Obama and Hillary Clinton has taken an increasingly nasty tone in the run-up to Wyoming's nominating contest on Saturday.
Obama, still the frontrunner in convention delegates won in the series of state-by-state contests - with Democrats having voted already in 40 of the 50 states - hopes to regain some momentum in the Wyoming caucuses after a strong Clinton comeback Tuesday in the Ohio, Texas and Rhode Island primaries, which plunged the tight race back into uncertainty.
Obama on Friday was forced to distance himself from a foreign- policy advisor who called Clinton "a monster" in an interview with The Scotsman newspaper, referring to the New York senator's campaign tactics.
Samantha Power, a Harvard University professor, called her remarks "inexcusable" and said she would resign from the campaign.
Obama on Wednesday had credited Clinton's "negative" campaigning with carrying her to recent victories, and challenged the former first lady to release her 2006 tax returns and documents that would shed light on her role in husband Bill Clinton's White House.
Clinton aides accused Obama's inquisitions of "imitating Ken Starr," harking back to the independent prosecutor that investigated Bill Clinton's indiscretions and scandals in the White House through the 1990s.
Clinton herself charged that she and Republican candidate John McCain, who secured his party's nomination on Tuesday, were the only candidates with enough national security experience to lead the country in a time of war.
"Senator McCain will bring a lifetime of experience to the campaign, I will bring a lifetime of experience, and Senator Obama will bring a speech he gave in 2002," Clinton said Thursday during a meeting with a group of retired military generals supporting her candidacy, deriding Obama's claim that he opposed the war in Iraq in a speech two years before his election to the US Senate.
Obama is expected to win the contests Saturday in Wyoming and Tuesday in Mississippi, after which the two candidates will have more than a month to prepare for the last big-state primary in delegate- rich Pennsylvania on April 22.