“Rick Perry was a train wreck in this debate. He flubbed his response on Romney flip-flopping. He got the first question tonight and stumbled. Good grief.” That’s the consensus in the mainstream and conservative media on last night’s event. What is remarkable is that this comment comes from Erick Erickson’s Red State, the head of the hard-right conservative Web site that hosted Rick Perry’s announcement speech. I suspect a lot of that reaction has to do with Perry’s position on in-state school tuition that is an anathema to the Tea Party base from which Perry draws his strength. Regardless of the rationale, it’s the equivalent of Tom Friedman turning on China or David Brooks turning on President Obama (which basically has happened).

Perry’s debate performances have made it increasingly hard for his fans in the right blogosphere to defend him.The same is true of donors and conservative power players. Can you imagine Sen. Jim DeMint (R-S.C.) endorsing Perry? Not unless he improves quickly and by a lot.And that in large part explains why in the Tea Party era Mitt Romney could very well win this thing.

It’s a truism that “you can’t beat something with nothing.” (Indeed, it’s Obama’s reelection strategy.) Romney is surviving (and in fact beginning to look like the front-runner he wasn't before Perry’s entrance) because no one has been able to challenge him successfully so far. Tim Pawlenty swung and missed. Rep. Michele Bachmann (R-Minn.) is bringing herself down with self-inflicted wounds and timid debate performances. And now Perry, nearly exhausted and somewhat bewildered by the end of last night’s debate, hasn’t been able to knock Romney off stride either. With all of these failed attempts to take him out, Romney is growing more confident.

The “nothing” part of the equation also includes quite a lot of good luck. There is a long list of people who didn’t run and may have been more effective combatants against Romney: Reps. Mike Pence (R-Ind.) and Paul Ryan (R-Wis.); Sens. Jon Kyl (R-Ariz.), John Thune (R-S.D.) and Marco Rubio (R-Fla.); Govs. Haley Barbour (Miss.), Mitch Daniels (Ind.), Bob McDonnell (Va.); and former governors Jeb Bush (Fla.) and Mike Huckabee (Ark.). This, of course, is how Bill Clinton got the 1992 nomination — the “top” candidates never emerged.

Then there is the economy. Jobs and the debt so swamped other issues that what seemed like a disqualifying factor, RomneyCare, became a nonfatal rather than a fatal flaw. And those issues happened to match Romney’s area of expertise: economic policy. Early in the race I pressed Romney supporters from his 2008 campaign — How is he going to win this? They didn’t bother justifying RomneyCare. They repeatedly said that if the race was about health care he would lose, but if it was about other things he’d win. That seems prescient right now..

A current Romney adviser told me last weekend that Romney had one more thing going for him: “He discovered he should just run as Romney.” Conservatives who find him lacking in fixed principles find that funny, I suppose. But he is who he is — a down-the-middle Republican, highly polished, very smart, able to attract experienced aides, capable of mastering material and working very hard, and, basically, a square. He stopped trying to convince voters he was the most conservative man in the race (his 2008 game plan) and trying to win the hearts of social conservatives. (He’s now playing to their minds and their overwhelming desire to beat Obama.)

My sense is that there are now only three things that can beat Romney, none of which is that likely. First — you knew this was coming — New Jersey Gov. Chris Christie could get into the race. He has the rhetorical skills and the governing record to challenge Romney. Second, Romney could make an egregious error and collapse. Frankly, it’s more likely that Christie would enter the race. Romney’s caution and penchant for preparation hugely reduce the chances of a gaffe so serious as to jeopardize his chances. Third, Perry could very quickly pull it together or Rick Santorum could emerge as the not-Romney. It’s a measure of how badly Perry has stumbled that the latter is now more likely.

The race is not over. But after nearly six weeks Romney is much more likely to win the nomination than he was before Perry entered on Aug. 13.

It’s good for Republicans that the leading candidates are being tested this severely. Imagine if Perry had rendered that debate performance against Obama. As my friend, the Republican operative, put it: If Perry improves he’ll get the nomination, and if not he won’t. Right now “not” is the safer bet.