Forget all the negative ads and countercharges. The race in Pennsylvania for the U.S. Senate comes down to what might be called the magic 10 percent.

That is the number of times that the challenger, U.S. Rep. Joe Hoeffel, says four-term incumbent Sen. Arlen Specter has voted to defy the Bush administration. One time in 10. The exact percentage can be argued, but the general point is right: Sen. Specter is a fairly reliable vote for the administration.

That 10 percent is magic enough for some labor unions and prominent Democrats -- Allegheny County Coroner Dr. Cyril Wecht among them -- who see Sen. Specter, 74, as a friend when they need one. He is, after all, a moderate pro-choice Republican who is always helpful to Western Pennsylvania and follows in the old tradition of GOP moderates from this state.

Unfortunately, that's a political position under siege both in the GOP nationally and in the commonwealth. Having made liberal a bad word, Republicans have drawn their big tent smaller and made it more hospitable to those such as Pennsylvania's junior senator, Rick Santorum.

Indeed, the lack of tolerance for any deviation from the party line explains why Sen. Specter had such a close race against the thoroughly conservative Rep. Patrick Toomey in the primary. It also explains why archconservative James Clymer is in this race under the banner of the Constitution Party (a Libertarian, Betsy Summers, is also a candidate). For some Republicans, there's nothing magic about that 10 percent: It means that Sen. Specter is not really one of their own.

This newspaper has always prized that independence, backing the senator in three elections and opposing him only in 1980 when hometown favorite Pete Flaherty unsuccessfully opposed him. We are not about to stop now.

The best argument for his staying on is his seniority, which puts him in line to be the chairman of the Senate Judiciary Committee. In that capacity, he would be in a position to block some of the ideologically extreme federal judges likely to be nominated by President Bush in a second term, some of them for the Supreme Court. Before the Post-Gazette editorial board, he promised that no extremists would be approved for the bench.

Such influence takes years to earn and a Sen. Hoeffel would be in no position to make much difference in the vital area of judicial appointments, even if he were to join the Judiciary Committee.

To be sure, Joe Hoeffel, 53, is a respectable candidate with a distinguished career in public service. A moderate Democrat, he has managed to win respect and support in Republican-leaning Montgomery County. Also a lawyer, he was a state representative from 1977 to 1984 and a county commissioner for eight years before being elected to Congress in 1998. Voters who are dead-set about seeing the Senate shift to Democratic control should go with Rep. Hoeffel.

But Sen. Specter, as a moderate Republican, is an endangered species. Even if he votes nine out of 10 times for the administration, we trust his word that the 10 percent of difference will be a brake on the worst excesses of a second Bush term, if it comes to that. And, if Sen. Kerry wins, he will still be a voice of reason. The Post-Gazette endorses Arlen Specter.