Sen. Arlen Specter (search), one of the Senate's (search) leading moderate Republicans, beat back a tough primary threat Tuesday, defeating a conservative congressman who lacked support from party leaders but gained momentum by casting the four-term incumbent as too liberal.

Specter, 74, prevailed against Rep. Pat Toomey (searchdespite a low turnout among Pennsylvania's 3.2 million registered Republicans that had been expected to aid the challenger.

The race, one of the GOP's most closely watched contests this year, tested the strength of the party's conservative wing. Specter's win also was a victory for President Bush, who endorsed Specter and is counting on his supporters to help him win carry a state he narrowly lost to Democrat Al Gore in 2000.

Specter, of Philadelphia, will face Democratic Rep. Joe Hoeffel in the Nov. 2 general election.