Hillary Clinton faces the daunting challenge of halting Barack Obama's momentum today, as voters head to the polls in three Democratic primary contests in which her opponent is considered a strong favorite.

The junior senator from Illinois, who has drawn even with New York's Clinton in the battle for delegates after sweeping five weekend contests, is a strong favorite to win today's voting in Maryland, Virginia and the District of Columbia -- the so-called Potomac primaries, for the river that touches all three.

Clinton needs to win in Virginia -- her best chance for an upset, analysts say.

"The expectations are that Obama will sweep the Potomac primaries, so losing Virginia would certainly blunt his momentum," said Brian Schaffner, a political scientist at American University in Washington.

Obama led Clinton in Virginia 60-38 percent, according to a SurveyUSA poll of likely primary voters released Monday night. The presumptive Republican nominee, Sen. John McCain of Arizona, led 48-37 percent over former Arkansas Gov. Mike Huckabee, with 7 percent for Rep. Ron Paul of Texas, a Green Tree native. The poll had a 4-percentage-point error margin.

The Clinton campaign is looking ahead to March 4, when delegate-rich Texas and Ohio hold nominating contests and where Clinton is deemed to have an early edge.

That strategy has risks, and the results could resemble those that brought down the campaign of former New York Mayor Rudy Giuliani, who held back resources in several early contests to make his stand in Florida.

"We saw how skipping several weeks of the campaign worked for Giuliani," Schaffner said. "I think it is fair to say that Clinton could really use a win in Virginia ... to keep Obama from entering March 4 with 10 straight wins." Continued...