How moderate would the other party's candidate have to be

[ian]

not too much.
Moderate dem vs. moderate repub, repub wins.
liberal dem (with exceptions, ie Boxer, Hil) against moderate repub, repub wins.

[ian]

Ian, are you saying that Boxer would be a good candidate for the Dems?

No, but I like her and would vote for her if she just happened to get the nomination; however, I would do anything to keep her from getting the nom, unless I hated everyone else she was running against (very possible).