His decision was a raw political calculation to advance himself and it backfired.
That is his 'mistake.'
Little Ricky made a calculated decision to back the liberal whose votes cancel out his own.
Little Ricky staked out the reputation of being 'the conservative Senator from PA' and Toomey -- a real conservative, threatened that. He had to stop him from winning because it just wouldn't do to be the '2nd most conservative Senator from PA.' He thought that it would undermine his run for higher office.
If there is a mistake, this is it:
He miscalculated the backlash from Toomey's supporters. If he had suddenly become the 2nd most conservative senator from PA, we would have two conservative senators and he would then be positioned as more 'reasonable' (earning him votes from pro life democrats) and would have earned the devotion of conservatives rather than their ire, and assuring his continued ascendency.
This assumes that he does hold conservative ideals and wants to see them advanced. I have serious doubts that this was anything more than a role that he was playing and that he couldn't care less about the issues that he claimed to share with conservatives.
Anyway, he is lost my trust and my vote.
This may sound crazy, but prior to Santorum supporting Specter, I would have walked across hot coals for the man, or at least done door to door campaigning for him - to me there's not much difference. He broke my heart when he supported Specter. I thought he was a true believer, only to discover he was just another power hungry politician.
I'd like to be able to say I won't give him my vote, but I probably will. And unless Toomey is running against him, I'll probably give financial support to him as well. Although I'm not going to give till it hurts, as he's proven he's just another Washington pol.