One reason many Republicans point to is the rise of Donald Trump. “He is sucking up all the sunlight and oxygen that budding candidates need to grow and develop,” said GOP strategist Alex Castellanos, who is not affiliated with any campaign, in an email exchange. “Because of Trump, candidates like Rubio have been starved of that.”

But Trump’s ascendance has affected all the Republican candidates. 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.”

If that is the case — if Bush is the biggest obstacle to Rubio’s rise — then Rubio has a lasting problem. The candidate who is in his way is the candidate likely to stay in the race the longest. That could frustrate the Rubio campaign for quite a while, regardless of those fundamentals.