The three people at the top of the ticket are Trump the super-rich showman that they would not be able to control, Cruz who embodies everything they vehemently despise and Rubio one-time TEA Party favorite.
Their only option is to informally support Hillary.
Rubio is part of the GOPe. His membership in the Gang of 8 leaves no doubt where his loyalties lie. Rubio gave the big middle finger to the Tea Party by not doing what he promised to get elected.