Arlen Specter, the senior senator from Pennsylvania and longtime endangered species, launched his 2010 re-election bid on the Senate floor yesterday.

Specter's ostensible target was Eric Holder, the Obama administration's nominee for attorney general; as the ranking Republican member of the Senate Judiciary Committee, Specter vowed - in notably harsh tones - to give Holder a very tough time during the upcoming confirmation hearings, just to ensure that Holder will not be merely a water carrier for the Obama White House.

But, between the lines, here's what Specter was really saying: "I'm up for re-election in less than two years, which means it's time for me to suck up to the right wing of my party. I've long had a reputation as a moderate Republican, but moderate Republicans are an endangered species, especially in the Northeast; basically, the only ones left in the Senate are me and the two ladies from Maine. The right-wing Pennsylvania voters almost booted me out four years ago - I barely survived the party primary that April, beating conservative Pat Toomey by only 1.6 percent - and those voters would love to get me in the 2010 party primary. So this means I've gotta start sucking up to them, prove to them that I can be tough on Obama, and play the obstructionist when necessary. Better start now."

Conservative activists require no less. One of the top activists, Patrick Ruffini, said as much the other day: "The GOP's number one priority politically is to set into motion a series of events that will make Obama look more ineffective, partisan, and unpopular than he is today...we need to set the stage for a unified and effective Republican opposition that will actually fight from top to bottom."

And so Specter is falling into line. One would certainly expect the ranking Judiciary Committee Republican to quiz Eric Holder closely during confirmation hearings; as I have written here, Holder was the Justice Department official, eight years ago, who signed off on President Clinton's notorious pardon of Marc Rich. But, until yesterday, Specter had only voiced mild skepticism about Holder. In his Senate speech on the Senate floor, however, Specter frontally attacked Holder, raising the possibility that Holder might turn out to be as bad as...Alberto "Gonzo" Gonzales, the notorious lackey for George W. Bush.

Specter's key passage: "Mr. Gonzales left office accused of politicizing the Justice Department, failing to restrain executive overreaching, and being less than forthcoming with Congress...I am convinced that many of attorney general Gonzales' missteps were caused by his eagerness to please the White House. Similarly, when Mr. Holder was serving as deputy attorney general to President Clinton, some of his actions raised concerns about his ability to maintain his independence from the president. I am prepared to give Mr. Holder a full opportunity to explain his past actions and convince the (Judiciary) committee and the Senate that his record warrants confirmation."

For Senate politesse, that's strong stuff. Holder hasn't even taken office yet, and already Specter is suggesting comparisons to Gonzo, and vowing to closely scrutinize the Clinton years (no doubt, with more vigilance than his Republican-led Judiciary panel ever scrutinized the Bush years).

But that's politics. The guy has to make nice with the right if he wants to survive a 2010 Republican primary; after all, Toomey and at least two other conservatives are weighing the possibility of assailing Specter on his right flank.

Moreover, Specter may have a problem with the Republican electorate. In Pennsylvania, the GOP runs a "closed" primary, which means that only registered Republicans can vote. As Rick Santorum, the former Pennsylvania senator, explained last week in a newspaper column, "Specter's biggest problem...may be that thousands of southeastern Pennsylvania Republicans who voted for him in 2004 can't vote for him in the (2010) primary. Why? Because they're no longer Republicans."

Exactly. I've got the figures here. Between November 2004 and November 2008, the statewide tally of registered Pennsylvania Republicans shrank by roughly 162,000. It's safe to assume that a huge share of those emigrants are moderates from the Philadelphia suburbs who got fed up with the Bush regime and the Republican Congress. Those are the voters that Specter needs to survive a primary; without them, the Republican primary electorate is disproportionately dominated by the conservative voters who have stuck with the party.

Santorum was right. Arlen Specter's top concern is not a challenge from the left, from Chris Matthews or some other Democrat. It's a heightened challenge from the right, from a candidate who can leverage the increasingly conservative primary electorate. Which is why Eric Holder will be the first item on Specter's to-do list.