COLLEGE PARK, Md. — The Obama campaign heads into a trio of primaries in the mid-Atlantic region today brimming with confidence, with one top supporter telling voters here that a victory over Senator Clinton could “put the nail in the coffin.”

With polls showing him ahead by nearly 20 points in the state, Senator Obama addressed an arena yesterday packed with more than 17,000 roaring supporters at the University of Maryland. He attributed the success of his candidacy to the backing of first-time voters across the country, saying their participation had confounded pundits and propelled his bid. “Young people are standing up,” he said to cheers. “A new generation is saying, ‘It’s our time.'”

The Illinois senator is coming off a weekend in which he swept four consecutive contests against Mrs. Clinton — in Louisiana, Nebraska, Washington state, and Maine — after the two candidates effectively split a nationwide vote on Super Tuesday.

Democrats head to the polls today in Maryland, Virginia, and Washington, D.C., and while Mrs. Clinton has made a significant push in Virginia, victories by Mr. Obama in all three primaries appear increasingly likely.

Mrs. Clinton’s efforts to regroup have been upstaged in the last day by news that she replaced her campaign manager. As the former first lady downplayed the move yesterday and insisted her campaign was in strong shape, Mr. Obama’s campaign was buoyed by two national polls showing him with a greater edge than she has in a potential general election matchup against the likely Republican nominee, Senator McCain of Arizona.

While Mrs. Clinton has defied expectations before in the campaign, most famously in New Hampshire, Obama supporters seemed to relish the notion that she is on the ropes.

“This is the state that tomorrow will put the nail in the coffin when we win this election,” Maryland’s attorney general, Douglas Gansler, told the crowd at the Comcast Center here before Senator Obama appeared onstage.

Mr. Obama stopped short of bold predictions, but he told reporters at a coffee shop in Silver Spring, Md., “I feel great about our prospects.”

Asked to respond to Mr. Gansler’s statement, a spokesman for the Clinton campaign, Blake Zeff, said: “Both campaigns know that this race is close and will go the distance.”

Enthusiasm for Mr. Obama peaked in anticipation of his appearances, as a line of students snaked across the campus toward the arena this morning. The crowd stood standing several minutes after Mr. Obama took the stage. “It looks like we’re having March Madness a little early,” Mr. Obama said. “By the way,” he added, “y’all can have a seat. Relax.”

In College Park and at a rally later in the day in downtown Baltimore, Mr. Obama tried to bat down the principal arguments that the Clinton campaign has made against his electability as a Democratic nominee.

“‘Obama is not tough enough to withstand the Republican attack machine.’ That’s what Senator Clinton said.” Mr. Obama began. “I have to explain to people: I’m skinny, but I’m tough.”

When the Illinois senator mentioned Mrs. Clinton’s name again, the crowd booed. “Now hold on,” Mr. Obama interrupted. “I want to give their argument a fair shot.”

He joked that he was also “wiry” and delivered a sly warning to his opponents: “Don’t mess with me.”

While applauding Mr. McCain’s “half century of service” to America, Mr. Obama said he was looking forward to “mixing it up” with the Arizona senator. “He’s the party of yesterday,” Mr. Obama said. “He’s the past. I’m the future.”

Mr. Obama appeared relaxed and confident throughout the day, interlacing his stump speech with jokes and asides. Here in College Park, he interrupted a section about President Kennedy when he saw the Maryland women’s college basketball team leaving the arena. “Is that the basketball team heading out? What’s up girls? Y’all going to the finals?” Mr. Obama said. After finding out that their record was 25–2, he added, to laughter: “If I had brought my sneakers, I’d suit up against you all. I still got game. I do.”

He stopped at the Mayorga Coffee Factory in Silver Spring on the way to Baltimore from College Park, where he greeted patrons and bought a half dozen cafes con leche and pastries.

Yesterday also brought word that a planned meeting between Mr. Obama and John Edwards at the former North Carolina senator’s home had been cancelled. Mr. Edwards met with Mrs. Clinton last week, and both candidates are aggressively courting his endorsement. “It will be rescheduled,” Mr. Obama said in Silver Spring. “We’re going to make it happen.”