Atrios makes my point for me. If you want to tell me how you're going to guarantee a left-wing challenger won't weaken Tauscher, or perhaps see her replaced by a moderate Republican, I'm all ears.
I have no idea what he means by "weaken" in this context. Sap her precious bodily fluids, perhaps?
But as for the second part, it's very simple Joe. Ellen Tauscher's congressional district is not Pennsylvania. John Kerry got 59% of the vote in 2004 in her district (51% in Pennsylvania). It's a safe Democratic seat, unlikely to go Republican absent scandal or a truly awful candidate. Pennsylvania is a barely blue leaning state which likes moderate Republicans in general, and Arlen Specter specifically.