"Presidents cannot say whatever's on their minds," he told a small crowd during a Tampa Bay-area rally. "You don't say it because it's wrong to do it — every society must be governed by rules of discourse. Once you lose the rules of discourse, you lose the discourse." "If he's our nominee, this is what the Republican party's going to be defined by. This is what it's going to mean to be a conservative," Rubio cautioned.