I would get my state party leaders together to get the word out. Stop Obama instead of duking it out for the futile effort to challenge McCain or influence the conservative agenda. Republicans know they would prefer to run against Hillary, don’t they?
They know that just about everywhere way more people are voting in Democratic caucuses and primaries rather than Republican caucuses and primaries due to Obamamania either for or against the front runner. And more Clinton supporters will support Obama if he wins than the other way around.
So given the inevitability of McCain’s victory, I would be encouraging Republicans, covertly of course, to storm the remaining Democratic primaries and vote for Hillary.
It’s not possible. Only a some primaries are open (I don’t know a specific number, but I think it’s a minority), and in the closed primary states - at least, as far as I know - you can’t just show up on the day of the vote and change your registration and then vote for the other party. Besides, large segments of the Republican Party are NOT convinced they want McCain to be the nominee, and they’re going to worry about that before they start trying to manipulate the Democrats.