Apparently these folks are just now realizing that Trump's nomination is the road to certain electoral ruin. But it will never happen. Changing the rules in the middle of the game is a losing proposition. The time to have stopped Trump from getting the nomination was in the primaries. Now it's too late.
A third party is the only practical way to preserve a Republican remnant against the deluge to come. And to have a chance of winning- of avoiding the alternative disaster of Hillary Clinton's election- it almost has to be Mitt Romney who leads the ticket.