WASHINGTON - President Barack Obama’s first pitch was way high and wide.

Pretty much all of Roy Halladay’s were spot-on as he struck out nine in his National League debut to help the Philadelphia Phillies beat the Washington Nationals 11-1 Monday.

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.”

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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.”