If saner heads prevail, this is the math: Trump + Rubio = majority.
Here's a thought of a source of votes. A lot of Sanders voters aren't ideologically extreme Socialists. They're mad as *ell, just like we are, and angrier still about hillary's campaign against Sanders. A lot of them will either stay at home or vote for Trump. That would not happen if the 'pub is anyone but Trump.
-- If saner heads prevail, this is the math: Trump + Rubio = majority. --
That's the math that the pundits are using. I don't think that math works except as a way to waste time on a talking point. Take any of the candidates out, and their votes disperse, some of them going to Trump, some leaving the party altogether.
I think the GOP has cooked its own goose, choked its own chicken. None of their players, not even the "stealth" one who pretends to be an outsider, is going to win.
Are you sure they wouldn't vote for Dr. Carson if he were the nominee? Trump has abused him as "low energy", but then so were Presidents Coolidge, Wilson, and Eisenhower -- Ike necessarily so in consideration of his heart condition.
There have been others who were not the sort of dynamo Trump publicly admires; and not all of his favored dynamos have been honest and effectual Presidents: Slick Willie, Lyndon Johnson, and Jack Kennedy all come to mind. JFK was very stylish and popular, like Trump, but his actual legislative legacy was not heavy, and he did not leave a high-water mark of utmost probity in his personal life.