The Republicans will not keep the Senate if they screw Trump or Cruz and go for Romney or .ryan as the nominee. Too many voters stay home.
It is likely that Trump enters the convention with more than enough first ballot votes for nomination. If he wins Florida or Ohio he will slug it out. If he wins both, it is over. Expect a vast push in Ohio and Florida for him and then he will roll through the East Coast all but untouched.
Until now everything was proportional. It is now big winner take all states.