In a 2 party system, yes it should
No it shouldn't. Bob Taft is one man; not the whole GOP. He's not even that representative of the GOP anyway. Simply because he is a terrible governor does not mean the whole party is terrible, or that the Democrats are any better.
Let's look at this another way. Kathleen Blanco of Louisiana is currently the lowest approved Democratic governor. Do you think the Lousiana Republican Party as a whole suddenly looks a lot more attractive?
While your first paragraph makes sense, its not the way our strict 2 party system works. If one incumbent becomes very unpopular, then at the next election his supporters usually don't vote for the other side, but stay home, and his opponent wins by default since all that matters is who has the most votes, even if very low like Clinton in '92 with 43%.
Obviously I don't like the Louisiana GOP, but even if Blanco's base hadn't been evacuated, I'd still say that her opponent has a much better shot at winning simply by not being Blanco.
Key difference: Blanco has a shot at a second term, while Taft is term-limited. For all of his problems, Taft will not be on the ballot, period. For that reason Taft's supporters will look to someone else that does share their beliefs and try to get that person elected, presumably it's Ken Blackwell, who currently leads in the GOP primary. If nominated, many conservatives disgruntled with Taft will enthuastically turn out for Blackwell. If a moderate managed to slip in, *then* they will likely stay home.
