After Iowa, he saw a quick surge in New Hampshire polling -- that stalled out. Across the board, Rubio's position is the same: If Ted Cruz and Donald Trump simply vanished, he'd be the front-runner. But they haven't vanished.
How has he performed?
Fine. He's capable, compelling, a decent campaigner and a pretty good debater, and his TV spots have been generally good.
But political watchers would be excused for expecting more. The candidate whose watchword is his youth and vitality has not brought a lot of that to the trail.
Then, of course, there was the debate in early February when his main strengths -- staying on message and being a good debater -- seemed to work against him.
Perhaps it won't matter -- but it was probably the worst moment of his campaign.
What are his strengths?
Rubio's main strength is that he is a candidate who is more than palatable to moderate and establishment Republicans while not being unacceptable to the conservative base. There hasn't been much oxygen for anyone besides Trump so far, and less still for anyone unwilling to try to push politics-as-usual in front of a bus.
Simply appearing to be the main contender for the moderate vote -- slipping past Jeb Bush's awkwardness and John Kasich's anonymity -- may end up being hugely advantageous if the race comes down to one candidate who's an outsider and one who isn't.
What's more, he has a compelling life story and the ability to tell it. He's naturally likable in the way that Bush, for example, isn't. That's not the most important criterion for being president, but it's an important one for running for the job.
What are his weaknesses?
Rubio doesn't seem to be running very hard. While most people probably don't care much that he's missed a large number of Senate votes as he's been campaigning, Rubio's campaign has also been criticized for its relative nonchalance. This may be a gamble that early-state visits don't mean much, which seemed to be correct in Iowa.
Part of that may be that his campaign is not raising as much money as many of his competitors. In the third quarter of 2015, he raised $5.7 million -- to Ted Cruz's $12.2 million, Bush's $13.3 million and Ben Carson's $20.8 million.
It's also hard to gauge the extent to which his involvement in the 2013 comprehensive immigration reform package will be a liability. Immigration has been central to the 2016 debate, thanks to Donald Trump's bear hug of it. Rubio has been hit on it repeatedly, but it could become more of a problem as the race narrows.
It's also hard to gauge, so soon before New Hampshire, the extent to which his debate performance might hurt him.
What would it take for him to win the nomination?
Rubio's best bet is to wait out the winnowing of the field, to be the last acceptable candidate standing in the rubble of the Republican establishment. If he remains viable as other more moderate candidates like Bush and Kasich drop out, he could conceivably consolidate their support -- perhaps even benefiting from Trump and Cruz splitting the anti-establishment vote as the race slogs on.
At this point, adding up Rubio's, Bush's, Chris Christie's and John Kasich's national support gets you 23.6 percent -- still almost 10 points behind Donald Trump and less than half of Trump and Cruz combined.
His campaign is talking about gradual growth in support over the short term, including that third-place finish in Iowa, followed by second in New Hampshire and first in South Carolina -- proceeding from there to roll up victories and strong finishes as the non-outsider pick.
Third in Iowa was the likeliest outcome. Second in New Hampshire seemed more likely right after Iowa; we'll see. From there? Rubio likely needs a lot more help from Trump and Cruz.