The verdict was clear when Marco Rubio announced his presidential run with a well-received speech in Miami last April: There was a new member of the top tier in the Republican race. "The official 2016 GOP field gets its first real contender," wrote fivethirtyeight. "Marco Rubio is now a first-tier candidate," said Bloomberg's Mark Halperin. Washington Post columnist Michael Gerson placed Rubio, along with Jeb Bush and Scott Walker, in "the first tier of plausibility."

It got even better from there. In June, the Washington Post declared, "Marco Rubio is now at the top of the Republican presidential field."

The polls seemed to support those bright assessments. Rubio went from seventh place in the RealClearPolitics average of polls, with 7.3 percent of the vote, on the eve of his announcement, to third place just two weeks later, with 10.6 percent. By early May he was in second place, behind Bush, with 14.3 percent. The idea that Rubio, Bush and Walker comprised the GOP's top tier settled into conventional wisdom.

But now that has changed. In the latest RCP average, Rubio has fallen back to seventh place in the race, with 5.2 percent of the vote — pretty much where he was before his announcement. "Rubio's springtime rise in the primary polls has faded, even as a nonprofit backing him has blanketed Fox News with TV ads about his foreign policy cred," writes National Journal. "He's basically back to where he started before a post-campaign launch bump."

National Journal and others note that Rubio's "fundamentals" are still good: He's still a likeable candidate, he still has the potential to attract Hispanic voters, he's still young, he's still a compelling speaker, he still appeals to big-money donors, etc.

That's the view in Rubio's circle. "We aren't paying any attention (literally zero) to the horserace numbers," said one member of the team, via email. "They are going to be all over the map. What matters to us is the fundamentals. Whether someone is supporting Marco today or not, what matters is how they FEEL about him. As in, do they have a favorable opinion of him, because favorable/unfavorable is always a leading indicator before vote choice."

The strategist pointed out that Rubio consistently has some of the highest favorable ratings and the lowest unfavorable ratings in the GOP field. To Rubio's team, that means he has lots of room to grow over the long course of a campaign. "With 17 candidates, no matter who someone is supporting, chances are their top choice is going to drop out," the strategist said. "So where do they go then? We think we have a good shot at getting them."

That's all perfectly reasonable, and things might work out that way. But Rubio is slipping now. Why? Is it just the normal up-and-down of a campaign, or does it indicate some deeper problem?

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.