Arlen Specter received a rating of "43" from the American Conservative Union last year. (For comparison, Rick Santorum received a "96"). Among Republican senators, only Olympia Snowe (RINO-ME) and Lincoln Chaffee (RINO -Gone!) received lower scores.
In case you're wondering, Ted Kennedy and Pat Leahy got "0" each, which matches their statistical likelihood of sobriety and honesty, respectively.