Placido Polanco hit a grand slam and drove in six runs, and Ryan Howard also homered for the Phillies, who got their push for a third straight World Series appearance off to a strong start.

"We definitely got started off on a good note today," Jayson Werth said. "Roy was magnificent. He was exactly what we expected."

Halladay (1-0) allowed one run and six hits in seven innings and settled down to dominate after the Nationals scored in the first. Ivan Rodriguez doubled to lead off the second, but Halladay then faced the minimum number of batters -- with help from a pair of double plays -- until the seventh, when he worked out of a two-on, one-out jam.

"It was a lot different," said Halladay, whose seven previous Opening Day starts came with the Toronto Blue Jays before being traded to the Phillies. "It's been fun for me. Nothing against Toronto, but it kind of gives you renewed energy coming over here. It's a team that wants to win and can win."

Halladay even helped himself at the plate with his second career RBI, albeit on a dribbler that traveled all of about 30 feet in Philadelphia's five-run fourth inning. He had plenty of support from a sellout crowd whose support was about evenly split.

"That was impressive. It felt like all of right field was only Phillies fans," Werth said. "This is starting to be our home away from home a little bit."

Obama received only scattered boos among thunderous cheers as he took the mound to mark the 100th anniversary of presidential first pitches. Not a natural baseball player by his own admission, the left-hander double-clutched before uncorking a wayward delivery that had third baseman Ryan Zimmerman standing and stretching his arm just to make the catch.

"It was high and outside. I was intentionally walking the guy," Obama quipped during an appearance in the Nationals' TV broadcast booth. "Fortunately, Zimmerman has a tall reach."

Obama wore a Nationals jacket but made an audacious fashion statement by donning a White Sox cap -- a nod to his favorite team -- as he walked to the mound.

"Bad move there," Washington manager Jim Riggleman said with a shake of the head.

"He said he wanted us to win," said Philadelphia manager Charlie Manuel, recounting Obama's pregame visit to the clubhouse, "as long as we didn't beat the White Sox."

On April 14, 1910, when William Howard Taft became the first president to take part in a first-pitch ceremony, The Associated Press story opened with the following sentence: "President Taft today enjoyed the novel experience of seeing the Washington American League team win a ball game."

Obama brought no such luck. The 1910 Washington team wasn't very good, and the 2010 National League version started off looking like the crew that lost 100-plus games in 2008 and 2009.

John Lannan (0-1) lasted only 3 2/3 innings, allowing five runs and seven hits with three walks and no strikeouts. He was done in by a morale-sapping fourth inning, when eight of the 10 batters he faced reached base. By then, Obama had left the ballpark.

"People are going to say what they want," Nationals right fielder Willie Harris said. "We can't really worry about that. We got to concentrate on the 25 guys that are in this locker room. We got to concentrate on sticking together, even if we do get out to a bad start."

Game notes

The Phillies broke a streak of four consecutive opening day losses. ... When the Nationals scored in the first, it broke Halladay's 24-inning scoreless streak over his last three starts with the Blue Jays. ... The loss ended the Nationals' majors-best seven-game winning streak from the end of last season. ... Zimmerman, Adam Dunn and Lannan were the only three Nationals starters from their opening day lineup a year ago. ... Rodriguez made his 19th consecutive opening day start, the longest streak in the majors. He had three hits, including a pair of doubles. ... Ian Desmond, Washington's new starting shortstop, made an error on the first ball hit to him. Only an alert play by first baseman Dunn kept a run from scoring as the Nationals went 6-3-2 to record the final out of the first inning. ... The Phillies were without 1B coach Davey Lopes, whose brother died in a house fire in Rhode Island on Friday. Manuel said he expects Lopes to be back after the current six-game road trip. ... Zimmerman was awarded his Gold Glove and Silver Slugger awards before the game. He and OF Matt Kemp of the Los Angeles Dodgers were the only National Leaguers to win both last year. ... Howard's homer reached the first row of the middle deck. ... Polanco's second career slam came in the seventh off Jason Bergmann. ... Commissioner Bud Selig tried to put a positive spin on the disproportionate number of Phillies fans in the ballpark. "I think it's great for the sport, I really do," he said. "There's enormous interest, obviously, in Philadelphia and the fact that people travel as much as they do now. ... It's only a testament to this game's popularity."