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Democratic rivals Sens. Barack Obama and Hillary Rodham Clinton squared off Tuesday in primaries in Maryland, Virginia and the District of Columbia, home to the White House, their long-sought prize

With 175 delegates at stake, Obama hoped to erode if not erase the lead Clinton has held since the campaign began.

The Illinois senator won a string of contests in all regions of the country over the weekend, routing Clinton in a Louisiana primary as well as caucuses in Nebraska, Washington state and Maine.

Early turnout in Virginia was reported high and city officials in the District of Columbia were hoping that a swath of new registered voters would show up at the polls. Maryland election officials were also projecting a strong turnout, particularly in the Democratic race.

The final Maine returns had not been tallied when Clinton's campaign manager announced she was stepping down. Coming several days after the former first lady loaned her own campaign $5 million, it was a fresh indication of the trouble the one-time front-runner is having fighting off Obama's strong challenge for the nomination.

Aides to the former first lady concede she is in the midst of a difficult period in which she could lose 10 straight contests. She is hoping to rebound on March 4, in primaries in Ohio and Texas, states where both candidates have already begun television advertising.

Clinton began the night with 1,147 delegates, to 1,124 for Obama. Both are far from the 2,025 needed to win the nomination at the Democratic National Convention this summer.

Among Republicans, Sen. John McCain, the faraway front-runner, hoped to rebound in Maryland, Virginia and the District of Columbia from a poor weekend showing. There were 116 GOP delegates at stake.

McCain lost caucuses in Kansas and a primary in Louisiana on Saturday to former Arkansas Gov. Mike Huckabee, his last remaining major rival. He won caucuses in Washington state.

The AP count showed McCain with 729 delegates. Former Massachusetts Gov. Mitt Romney, who dropped out of the race last week, had 288. Huckabee had 241 and Texas Rep. Ron Paul had 14.

It takes 1,191 delegates to clinch the nomination, and McCain appears to be on track to reach the target by late April.

Obama has campaigned before huge crowds in recent days, and far outspent his rival on television advertising in the states participating in the regional primary in Maryland, Virginia and the District of Columbia.

He began airing commercials in the region more than a week ago, and spent an estimated $1.4 million. Clinton began hers last Friday, at a cost estimated at $210,000.

With Clinton facing a series of possible defeats, and Obama riding a wave of momentum, the two camps debated which contender is more likely to defeat McCain in the general election.

An Associated Press-Ipsos poll found Obama with a narrow lead over the Arizona senator in a potential match-up, and Clinton running about even.

"We bring in voters who haven't given Democrats a chance" in the past, said Obama pollster Cornell Belcher, citing support from independents.

Mark Penn, Clinton's chief strategist, countered that she holds appeal for women voters and Hispanics. "Hillary Clinton has a coalition of voters well-suited to winning the general election," he said.