He won't be forced. He can nominate conservative after conservative, and let the post remain empty as the liberal vote them all down. He won't, though, because in his heart he'd prefer "moderate" (aka they'll vote liberal more than conservative) justices anyway.
Well, that’s an option I hadn’t considered. True. But this COULD be a terrific tactic for a conservative who could paint the libs as, well, libs. But McCain won’t do that. He’s too “principled.”