As the deadline to register for the Pennsylvania primary passed yesterday, thousands of voters changed their status to the Democratic Party, inching the number of registered party members in the state north of four million for the first time in state history. Aside from presidential politics, the registration boom is part of a continuing shift in certain areas for reasons that will outlast both Hillary Clinton and Barack Obama.

In 2006, Democrats contested three Republican-held seats in the Philadelphia suburbs, beating GOP Reps. Mike Fitzpatrick and Curt Weldon and narrowly missing out on the chance to knock off Rep. Jim Gerlach. Freshmen Democrats Patrick Murphy and Joe Sestak are favored to win re-election in their suburban districts, which for generations has voted Republican, though the GOP still holds out hope.

But the areas are changing, and Gerlach, the sole survivor of the 2006 massacre, could find himself in more trouble as more suburban voters get used to casting ballots for Democrats. Several counties that have experienced the highest increase in Democratic registration are in those Philadelphia suburbs, the Associated Press reports, and though Republicans still maintain registration edges in those counties, the advantage is shrinking.

Murphy, who represents Bucks County and a few neighborhoods in the northern part of Philadelphia, beat Fitzpatrick in the state's Eighth District by a mere 1,500 votes two years ago. Sestak, a retired naval rear admiral, has Delaware County and beat Weldon in the Seventh District by twelve points after the Republican's home was raided by the FBI just days before the election. The lone Republican, Gerlach, won a 3,000 vote victory against a Democratic challenger who outspent him in the more exurban Sixth District, which extends west to Chester County.

Murphy and Sestak are likely to be safe, especially during a presidential election year: John Kerry and Al Gore won both districts in 2000 and 2004, though by narrow margins, and turnout is expected to be high again this year. Gerlach, who should be a top Democratic target, faces a retired businessman in Bob Roggio after his 2004 and 2006 opponent, businesswoman Lois Murphy, declined to run again.

Roggio is unlikely to raise the impressive sums Murphy did in both her contests, but he's an experienced field operative, having run Kerry's Philadelphia campaign in 2004 and Senator Bob Casey's efforts in suburban Philadelphia in 2006. While Gerlach may be able to win a more comfortable re-election this year, his long-term prospects don't look as good, thanks to booming Democratic registration.