Byron York asks why Rubio’s support keeps declining, and offers a possible answer:

For Rubio, though, something else is happening. “Rubio has an even bigger challenge,” said Castellanos. “Jeb Bush is beer, and Rubio is lite beer. [Bush] has a more mature brand in his category. Many voters see Rubio, charismatic as he is, as Jeb Bush without the experience. In NASCAR terms, Rubio is drafting behind Jeb’s car and only has a chance if Jeb’s car hits the wall and clears the way ahead, so voters can pay attention to him.”

I have assumed from the start that Rubio’s candidacy made no sense, especially when Bush was already going to be taking up the space that Rubio wants to occupy. This interpretation supports my view that Rubio isn’t likely to be competitive in the primaries next year and was never going to be so long as Bush was also in the race. The reason why many voters would see Rubio as “Bush without the experience” is that their domestic policy views are very similar, their political careers in Florida were closely linked, and their rhetoric on immigration and foreign policy has been virtually indistinguishable in the past. To the extent that Rubio has separated himself from Bush in these areas, he has done so by running away from his previous position on immigration and by emphasizing his hard-line foreign policy views. In the process, he has made himself seem both more opportunistic and more ideological than Bush, and that doesn’t appear to be winning him much support. The question to ask about Rubio’s candidacy isn’t, “What happened to him?” but rather “why did he decide to run at all?”