As recently as five months ago, Mitt Romney boasted about having created “tens of thousands of jobs” at his vulture-capital fund.

“When I was at Bain Capital, we invested in about 100 different companies. Not all of them worked…. But I’m very proud of the fact that I learned about how you can be successful with an enterprise, why we lose jobs, how we gain jobs and overall, in those 100 businesses we invested in, tens of thousands of jobs, net-net, were created.” [emphasis added]

That was the same line Romney took three months ago, in early October.

By December, Romney had come up with a new number.

“In the real economy, some businesses succeed and some fail,” said Romney. “That’s how that works and you try and encourage the more successful and fortunately for many people, tens of thousands of jobs, actually over a hundred thousands of jobs were created by the investments that we were able to help make.” [emphasis added]

The 100,000 figure quickly became the standard talking for Romney and his supporters, but it’s been definitively debunked. The number simply has no basis in reality, leaving the Republican frontrunner with a weak foundation for his entire campaign.

Which leads us to the new/old line, which Romney shared on CBS this morning:

“People here in the state know that in the work that I had, we started a number of businesses, invested in many others, and that over all created tens of thousands jobs so I’m pretty proud of that record.” [emphasis added]

So, over the course of a few months, Romney went from five-figure job growth, to six-figure, and then back to five-figure. Anyone who finds the former governor’s line persuasive isn’t paying close enough attention.

Why is Romney proving to be so incoherent on what’s supposed to be his top issue? After all, as we discussed yesterday, this is supposed to be the raison d’etre of his entire campaign: elect him president, the argument goes, because he created so many jobs in the private sector. If this is his top selling point, why can’t Romney keep his own story straight? What’s with all the fuzzy math?

The answer is, Romney is trying to turn his private-equity firm into something it’s not. Bain Capital and its executives weren’t in the job-creating business — its purpose was to make money for its investors, not grow businesses and create jobs. What’s wrong with wealth creation? In theory, nothing. The problem, though, is when Romney decides to describe his firm in ways that are at odds with reality.

As Glenn Kessler explained, the Bain prospectus “never mentions ‘jobs,’ ‘job,’ or ’employees.’” Ezra Klein added, “Those simply aren’t the objective. Sometimes, in fact, they’re collateral damage.”

Again, this is just how private-equity firms work. It’s a feature, not a bug.

But Romney wants to repackage reality in a way voters might find more appealing, since the truth doesn’t quite work. The result is a series boasts that completely fall apart under scrutiny.

Or as Ezra concluded, “Romney invited the electorate to judge his economic chops by judging his success at something he wasn’t trying to do and that isn’t relevant to the policies he would pass as president. The more energy he invests in this narrative, the larger his eventual losses will be.”

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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.