More specifically, both Cruz and Rubio have weaknesses that could hold them back.

Jeb Bush’s Super PAC still has tens of millions of dollars it could spend trashing Rubio’s reputation — and other attacks might not be swatted away so easily as the one on Rubio’s voting record. Other candidates could hurt Rubio by attacking his past support for comprehensive immigration reform. Plus, there has been some chatter that Rubio isn’t doing a particularly good job building a campaign operation, particularly in the early voting states. And even if Bush vanished from the race and Rubio picked up all his support, he’d still be trailing both Trump and Carson.

For Cruz, the main problem is, well, that the theory assumes Trump and Carson will vanish. That still could happen, and the history of non-politician candidates suggests it will — but the clock is ticking. Trump’s ability to dominate headlines between debates, and the enthusiasm shown for Ben Carson on social media, could keep them around for some time. Cruz has spent years positioning himself to run against the Washington establishment, but his credentials as a political outsider are obviously inferior to theirs.

While watching this debate, though, it was easy to imagine both Cruz and Rubio rising above everyone else — and battling it out for the nomination in the end.