After five debates in six weeks, these get-togethers for the Republican presidential candidates were quickly becoming sleep-inducing. Last night in Las Vegas, though, was at least lively — with time running out, someone lit a fire under the candidates’ feet.

Ultimately, there was more heat than light, and viewers didn’t actually learn much in the way of substance. There was, however, one revelation that might linger.

About mid-way through the event, CNN’s Anderson Cooper asked Rick Perry about Texas having one of the highest rates of uninsured children in the country. Perry started to answer, before ignoring the question and launching an attack on Mitt Romney for having hired undocumented workers.

“[W]e have a 1,200-mile border with Mexico, and the fact is we have a huge number of illegals that are coming into this country,” Perry said, adding, “[T]hey’re coming here because there is a magnet. And the magnet is called jobs. And those people that hire illegals ought to be penalized. And Mitt, you lose all of your standing, from my perspective, because you hired illegals in your home and you knew about it for a year.”

This led to a very heated exchange — easily the most contentious moment of the year to date — with Perry and Romney talking over each other and Romney putting his hand on Perry’s shoulder.

Eventually, Romney explained himself. Here’s the key quote, with video by way of TPM:

The former governor said he’d hired a lawn company that, unbeknownst to him, hired undocumented workers. “We went to the company and we said, ‘Look, you can’t have any illegals working on our property. I’m running for office, for Pete’s sake, I can’t have illegals!” Eventually, Romney said, he fired the company when it was “pointed out” to him that it still employed undocumented workers.

In terms of fact-checking, Perry happens to be correct. The Boston Globe reported on Romney indirectly hiring immigrants who entered the country illegally, and the governor did nothing. A year later, the Globe ran another story, which led Romney to act.

In the larger context, as controversies go, this isn’t terribly interesting. What matters, though, is how Romney explained himself: he told his lawn company not to use undocumented workers because he’s “running for office.”

Romney, in other words, offered some accidental candor last night — he doesn’t care about breaking the law or paying undocumented workers; he cares about his political ambitions.

None of the other candidates seized on this last night — though they may very well pounce today — but Romney made it quite clear that he puts political ambitions over principles. Suspicions that Romney is a shallow, calculating politician, who cares more about electoral objectives than anything else, were confirmed when he accidentally spoke the truth.

It’s a safe bet we’ll hear this quote again soon.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.