, the Republicans' presumptive nominee, also eyed the election contests in Virginia, Maryland and Washington, D.C., to rebound from embarrassing weekend losses to rival and show that he can rally the party's conservative base behind him.
In the Democratic race, Obama, locked in an epic battle with Clinton for the party's nomination, was counting on those three primaries to erase Clinton's slim delegate lead for an edge in a contest that could stretch to the Democrats' national convention in August.
Black Democrats, who have supported the Illinois senator in his bid to be the first black U.S. president, are expected to factor large in the three so-called Potomac primary races, named after the river that touches all three localities.
Obama's nearly 5-1 margin among blacks has forced Hillary Clinton to fight for a voting bloc once felt to be securely hers, reports CBS News correspondent Dean Reynolds.
Over the weekend, Obama secured a clean sweep in five races: Louisiana, Washington state, Nebraska, Maine and the U.S. Virgin Islands.
Clinton's losses, coupled with her decision to replace her campaign manager with a longtime aide and Obama's infusion of new funds, fueled talk that the New York senator's campaign bid was falling on tough times. Clinton dismissed such speculation Monday.
"I'm still ahead in the popular vote and in delegates," Clinton told reporters at a Maryland campaign stop, though the numbers do not fully support that statement.
Delegate estimates compiled by CBS News gave Obama a slim lead after a string of convincing weekend victories in Louisiana, Nebraska, Washington, the Virgin Islands and Maine. Including superdelegates, the Democratic elected officials and party leaders who have a vote at the party's convention, Obama has 1,139 delegates while Clinton has 1,132. A candidate needs 2,025 delegates to secure the nomination. (View the Democratic delegate scorecard.)
Clinton, who is seeking to become the U.S.'s first female president, is also looking for a big rebound in the high-stakes March 4 primaries in Texas and Ohio. Both candidates said Monday they were launching ads in those states.
Obama is riding a wave of support after winning more states than Clinton in the 22 contests on Super Tuesday last week and then sweeping the weekend races by wide margins. The remainder of the February calendar looks favorable to him, including next week's contests in Wisconsin and his native Hawaii.
Obama was loose and enjoying himself Monday in front of another huge crowd - looking every bit the man who swept all the contests this past weekend, reports Reynolds
Clinton and Obama largely agree on the major issues, so both are increasingly emphasizing who would be the stronger candidate in the general election against McCain.
An Associated Press-Ipsos poll shows Obama would narrowly defeat McCain if the presidential election were being held now. If Clinton were the Democratic nominee, she and McCain would run about even.
Obama said Monday he is the candidate who can lead the country out of a long period of divisive and ineffective government, a theme he increasingly uses against Clinton, who was first lady for eight years.
"We need something new," he said, dismissing Clinton's suggestions that he is not tough enough to handle the rigors of the presidency.
But the Democratic nomination is far from decided, with weeks or months of campaigning still ahead. Clinton is an experienced campaigner certainly capable of pulling off more surprise wins, as she did Jan. 8 in New Hampshire. The AP-Ipsos poll still shows Clinton leading Obama 46 percent to 41 percent nationally among Democrats.
Clinton said she never expected to do well in any of the weekend contests, and her campaign predicted that while Obama may take the spoils in February, March would be her month.
In the Republican race, McCain challenged the notion he is struggling to rally conservative critics as he picked up the endorsement of Gary Bauer, an evangelical leader.
"We're doing fine. We're doing fine," McCain told reporters in Annapolis, Maryland.
Bauer, who unsuccessfully sought the Republican presidential nomination in 2000, said in an interview with the AP that he wanted to "unite conservatives" and that McCain's anti-abortion stance makes him a better choice than Clinton or Obama.
McCain is "going to need every last one of the conservative activists," he said.
McCain lost to preacher-turned-politician Huckabee on Saturday in Kansas and Louisiana, although he won narrowly in Washington state. Huckabee's weekend victories highlight the difficulty the veteran Arizona senator faces in convincing the party's core right-wing blocs that he is one of them.
Still, McCain is all but assured his party nod after rolling up huge numbers of delegates to the national convention. CBS News delegate estimates McCain has won 705 delegates so far, giving him nearly 60 percent of the 1,191 required to secure the nomination. Huckabee, a former Arkansas governor, is far behind, with 199 delegates. (View the Republican delegate scorecard.)
McCain appeared likely to rebound on Tuesday in the next Republican contests. Polls conducted last week by Mason-Dixon Polling & Research Inc. showed the Arizona senator leading Huckabee by nearly 30 percentage point margins in both Virginia and Maryland. The Republicans also compete in Washington, D.C., on Tuesday.