“because when the electorate is faced with a choice between a liberal Democrat and liberal Republican, they go with the real deal.”
Perhaps you might consider Rick Santorum...he was a great conservative and lost rather badly in PA to Casey.
Santorum similarly pissed off the Conservative base by endorsing Arlen Specter over Pat Toomey (how’d that work out for him ? Arlen sure helped, not). Bob Casey, Jr. (who had won office in PA statewide three times) marketed himself as the second coming of his dad, who was revered by Social Conservatives for his pro-life activism (who didn’t just talk the talk, but walked the walk). A lot of old timers thought Casey, Jr. WAS his dad. Add up those two and an overall decline in GOP turnout, and the Casey win wasn’t a huge surprise.