Obama and Clinton HQs open in Phila.

The front door had no sign, and the newly installed banks of computers have no Internet access.

But hanging in a cavernous fourth-floor room, lit by skylights today, was a large banner that declared "Yes, we can!"

Barack Obama's Philadelphia headquarters near 15th and Sansom Streets was bustling by 7 o'clock this morning in offices that once served Gov. Rendell's 2002 campaign.

Across town, Hillary Clinton's outpost above an abandoned nightclub at Spring Garden and Delaware Avenue was markedly quiet. A dozen signs posted in the second-floor windows were among the only evidence that the campaign occupied the offices.

"People are in the field, getting ready for the Chelsea event later at Penn," said Mark Nevins, communications director for the Clinton campaign.

Chelsea Clinton is scheduled to appear for a outdoor rally at 4:30 p.m. at the University of Pennsylvania, at the Wynn Commons outside of Houston Hall, at 3417 Spruce St.

Clinton campaign activities were scattered yesterday across the city. Dozens of volunteers manned phone banks at the American Federation of Teachers on the 1800 block of Chestnut Street.

Now that Hillary Clinton's fortunes have been dramatically revived with wins last night in Ohio, Texas and Rhode Island, both camps are gearing up for a crucial showdown in Pennsylvania on April 22.

At Obama headquarters, enthusiastic voters began walking in shortly before noon to volunteer their time.

"We were up late last night and early this morning," said Leslie Miller, director of communications for Obama's Philadelphia campaign.

This morning, about two dozen workers - mostly young and casually dressed - were busy about the office.

Miller said a dozen more offices would be opening across the state by the weekend, as workers and equipment come in from Ohio, Texas and Vermont.

"We're in for the long haul," Miller said. The campaign is preparing to work here for the next seven weeks.

Sean Smith, a senior campaign worker, said workers had already been canvassing neighborhoods and registering voters.

About 41,000 Pennsylvania Republicans and independents had switched to the Demoocratic Party this year, Smith said, "we believe, to vote for Barack Obama."

The Obama campaign effort here probably won't be fully staffed, however, until next week, after contests in Wyoming and Mississippi, he said.