I think it goes beyond your thoughts. Who is the most vulnerable to the pent-up anger of the electorate in general? Focus more on the biggest threat and at the time it was Bush because of all the money.
Now Rubio finds himself in the establishment hot seat with the exit of JEB!. When you figure that over 60% of the SC vote went to insurgents and Cruz neck and neck with him, I see a multi-front barrage coming to Rubio’s real record, despite his ability to try and paint a far different picture.
The Cruz/Trump voters will not go to Rubio. When those two patch up their differences the next POTUS will be decided.