As Krugman mentions, Marco Rubio is leading in the betting markets, if not the polls. (Unlike Krugman, I give a lot of weight to the betting markets; I find them to be much more predictive than polls. For example, when the polls got all excited about Newt Gingrich in 2012 the betting markets stayed with Mitt Romney.)
Right now it looks like Donald Trump and Ted Cruz are the frontrunners -- both anathema to the party establishment. While Jeb Bush and John Kasich look like they're both on life support, Chris Christie actually has a chance of a good showing in New Hampshire. That could really muddy the race in the establishment "lane." But at some point, the party elders have to get behind one candidate and it could very well be Rubio.
So let's assume the party says, "Oh alright. Fine. Rubio." Then what? What state does a Marco Rubio candidacy take that Romney didn't in 2012? His home state of Florida? Okay. That would give Rubio 29 Electoral votes that Romney didn't get. So the election would look like this:
Hillary Clinton 303
Marco Rubio 235
What if Rubio picks either Kasich or Sen. Rob Portman to be his running mate and the GOP takes Ohio as well? Here's what you'd get:
Let's go nuts and say the popular vote is really, really close and the Republicans capture Virginia this time around. Now what?
So even if Rubio steals the three closest states President Obama won last time he'd still come up short. He would need one more, like Colorado, to put him over the top. That would be a "heavy lift," as they say in Washington nowadays.
Oh, alright. Fine. Rubio. I'm in!