UNLESS YOU'RE heavily

sedated, you know by now that Pennsylvania will host the next and longest brawl between Hillary Clinton and Barack Obama, what Chris Matthews called "seven weeks of thunder" before our April 22 primary.

But here's a thought: For Obama, the primary is actually March 24.

I exaggerate, but only a little.

That's the deadline to register to vote in the primary election, and Obama could deliver the knockout punch that has so far eluded him if he gets a big push from the independent and first-time voters who've brought such energy to his campaign.

But in Pennsylvania, they can't vote unless they're registered Democrats by midnight, March 24.

That gives the Obama campaign just 18 days to get every independent, every Green, every Ralph Nader-ite, every college kid or alienated yuppie, even the odd Republican yearning for change, to register as a Democrat.

Believe me, the Obama campaign knows this.

They like to think they can beat Hillary even among the state's existing Democratic electorate.

But so far, Obama has won primaries in only three states - Connecticut, Delaware and Maryland - plus the District of Columbia where only previously registered Democrats could vote.

One of Obama's best field people, Jeremy Bird, has been here for two weeks, and the campaign will be on a registration binge until the 24th.

"The next three weeks are critical to determining the outcome of this race," said Chester County Obama supporter Dan Wofford.

Heaven knows there are waters for the Obama team to fish in.

In Philadelphia, 92,000 people are registered as independents or third-party members, 10,000 of them in just two Center City wards.

A lot of those folks are surely Obama supporters who are currently ineligible to vote for him.

"This is an issue I've been talking about for a long time, the importance of getting new independent voters to re-register as Democrats," said Obama supporter Marc Stier, a founder of the grassroots group Neighborhood Networks.

"If the Obama campaign is smart, they'll try to address it and it will help all the progressives," Stier said.

Philadelphia is a city with a tradition of voters switching parties to vote in primaries. In 1987, former mayor Frank Rizzo got tens of thousand of conservative Democrats to become Republicans to help him in a primary battle.

In 1991, Republican Sam Katz got liberal Democrats to hold their noses and embrace the GOP, at least for one primary vote.

Green Party members might be seen as potential Obama recruits, but the Greens plan to run their own candidate for president, and they're concentrating on circulating petitions to get on the Pennsylvania ballot.

Nothing would prevent Green Party members from doing that while briefly stepping into the Democratic Party to vote for Obama, but party spokesman Scott McLarty hopes they won't.

"It's their right to do that, but we discourage them," McLarty said yesterday. "We have four outstanding candidates running for president, and we'll chose one of them at a convention in July."

Their candidate won't be Nader, by the way. He's announced

he'll run as an independent.

Obama's Pennsylvania spokesman, Sean Smith, said the campaign will even target registration efforts at Republicans.

"Typically there are certain areas or pockets where Republicans will vote for Democrats in a given election," Smith said. "You can see that by analyzing election results, and we're going to target those areas."

In another odd twist, right-wing talk-radio host Rush Limbaugh has been urging Republicans to vote for Clinton to prolong the Dems' intraparty battle. The Obama team will have to be careful about which Republicans they try to switch.

Getting Republicans and independents on board in an aggressive registration effort is important, said one Obama activist, not just to win the primary, but to show that the Obama crusade can expand the party and change the nature of the Democratic electorate.

A lofty goal.

They have 18 days.*

Send e-mail to daviesd@phillynews.com or call him at 215-854-2595.