The numbers don’t look good for Sen. Arlen Specter’s 2010 re-election race.

The moderate Pennsylvania Republican will face a tough primary challenge next year from former Rep. Pat Toomey, and GOP enrollment is declining in the Keystone State — especially in Specter’s political stronghold.

According to election data tabulated by Roll Call, Specter’s suburban Philadelphia base has seen the sharpest drop in Republican registrations in the state — numbers that collectively exceed his margin of victory over Toomey in the 2004 Republican primary.

A simultaneous spike in Democratic enrollment likely means that moderate Republicans switched their registrations over the past four years. The result is bad math for Specter, who will likely face a more conservative GOP electorate in the 2010 primary than he did in 2004.

Specter did receive some good news this week when anti-abortion activist Peg Luksik announced she will run against him and Toomey in the primary. Luksik, who has run statewide three times, could siphon conservative votes away from Toomey, who has yet to officially announce his candidacy but is expected to do so. Specter’s primary challenger from 1998, Larry Murphy, has also announced his candidacy again for next year.

Whoever wins the GOP primary will likely get a hefty challenge from Democrats, who plan to target the seat in 2010. Former National Constitution Center President Joe Torsella has already announced his candidacy, and state Auditor Jack Wagner, Reps. Patrick Murphy, Allyson Schwartz, Joe Sestak and state Rep. Josh Shapiro have also been mentioned as possible Senate candidates. National Democrats concur that any of them would have a better chance of beating Toomey in the general election than Specter, but that the seat will be targeted no matter what.

Specter defeated Toomey by 17,000 votes in the 2004 primary, and since then Republican registrations have decreased by 83,412 in Philly and the four populous counties that comprise its suburbs — Specter’s base. Pennsylvania has a closed primary system, so only enrolled Republicans will be able to weigh in on the Specter-Toomey race.

Specter’s campaign manager, Christopher Nichols, said that he will make an effort to bring former and new Republicans back to the GOP side of the aisle.

“As he always has, Sen. Specter is working to grow the GOP,— Nichols said. “He is working very hard to get more people across the state to register or re-register Republican because that’s a crucial step to getting back in the majority. He believes strongly in our two-party system of government, and for that we need a vibrant GOP.—

But with more than a year until the primary, Specter might have to reverse six years of GOP defections to win both the primary and general election. The remaining members of the Republican Party in southeastern Pennsylvania are more conservative, said David Madeira, a conservative activist who supports Toomey.

“It’s become more conservative. It’s sort of purged itself of its liberal tendencies,— Madeira said. “On the primary level, the balance has tipped towards conservatives.—

In Montgomery County alone, Republicans lost a net 17,750 registrations from 2004 to 2008. Specter defeated Toomey by 14,718 votes there in 2004, according to data from the Pennsylvania Department of State. Democrats, however, had a net gain of 71,614 registrations in the county over those four years.

Philadelphia County lost an even larger number of GOP voters — 22,203 — from 2004 to 2008, while Democratic registrations increased by 152,814. Specter defeated Toomey in that county by 7,050 votes.

The other populous counties surrounding Philadelphia, including Delaware, Bucks and Chester counties, also saw GOP registrations drop by at least 11,000. Specter won all three of those counties by fewer than 8,000 votes in the GOP primary. Those three counties, plus Montgomery and Philadelphia, saw the largest drop in Republican registrations from 2004 to 2008 in the entire Keystone State.

The Pennsylvania GOP will have a challenge when it comes to retrieving former Republicans. According to a recent survey, the majority of those voters were longtime members of the party before they became Democrats.

In a recent Muhlenberg College poll of 400 Pennsylvania voters who switched their registration from Republican to Democrat in 2007 and 2008, 53 percent reported that they had been members of the Republican Party for at least 20 years. The survey, taken Nov. 19-26, had a margin of error of 4.5 points.

But if they’ll switch back for anyone, it might well be for Specter. The survey also found that 67 percent of the party-switching respondents supported abortion rights, while only 19 percent were anti-abortion. Specter is one of only a few Republicans in the Senate who supports abortion rights.

Republican consultant Vince Galko, who ran former Sen. Rick Santorum’s (R-Pa.) 2006 campaign, said he thinks the state GOP’s dwindling ranks will rebound.

“I think the registration deficit will shrink in the next couple months,— Galko said.

Galko, who also used to work for Specter, said the Senator will have to mount a campaign to switch Democratic voters back to Republicans for the primary.

It would not be the first time Keystone State voters have seen such a drive. When Gov. Ed Rendell (D) ran for his first term in 2002, he implemented a campaign to get moderate Republicans to change parties in order to vote for him in the closed Democratic primary over now-Sen. Bob Casey (D). Although the number of crossover voters as a result of the campaign could never be determined for sure, local operatives say Rendell re-registered 40,000 Philadelphia area Republicans as Democrats.

A second wave of voter registration switches occurred before April 2008, when many Republicans switched to vote in the Democratic presidential primary — a contest that then-Sen. Hillary Rodham Clinton (D-N.Y.) won resoundingly.

“Most of those voters switched for Hillary Clinton and not Barack Obama, so I think they’d be willing to come back over to the [Republican] Party,— Valko said.