Mitt Romney tends to shine in Republican presidential debates in large part because he generally seems prepared. Putting aside whether his answers are truthful or sensible, the former governor’s answers are generally polished — he gives the appearance of someone who at least tried to do their homework.

It was bizarre, then, to see Romney stumble so badly when the issue of his hidden tax returns was raised during Fox News’ debate last night. Did he not realize the subject might come up?

The Wall Street Journals Kelly Evans said, “Governor Romney, Speaker Gingrich, Senator Santorum and now vocally tonight Governor Perry are calling for you to release your tax records. The Obama campaign is asking for the same thing. Governor, will you release your income tax records?” Here’s his response:

“You know, I looked at what has been done in campaigns in the past with Senator McCain and President George W. Bush and others. They have tended to release tax records in April or tax season. I hadn’t planned on releasing tax records because the law requires us to release all of our assets, all the things we own. That I have already released. It’s a pretty full disclosure. But, you know, if that’s been the tradition and I’m not opposed to doing that, time will tell. But I anticipate that most likely I am going to get asked to do that around the April time period and I’ll keep that open.”

Evans followed up, asking if that means the tax records will be released around April. Romney added:

“I think I’ve heard enough from folks saying, look, let’s see your tax records. I have nothing in them that suggests there’s any problem and I’m happy to do so. I sort of feel like we are showing a lot of exposure at this point. And if I become our nominee, and what’s happened in history is people have released them in about April of the coming year and that’s probably what I would do.”

The transcript doesn’t do it justice. Romney appeared to be dissembling badly, and it wasn’t long before the DNC was using it against him, releasing this video overnight:

On the substance, Romney has said previously he would not release the tax returns. Last night seemed to offer a different answer, though it was hard to tell exactly what the frontrunner is now prepared to do. I think the bottom line is that Romney won’t rule it out, but won’t disclose anything until after GOP voters give him the party’s presidential nomination.

We can only speculate as to exactly why the former governor is so reluctant, but it’s a pretty safe bet that Romney doesn’t want the public to know he pays a lower tax rate than middle-class workers. Because Romney still collects seven-figure checks from his vulture-capitalist firm, he benefits from the “carried interest” loophole, which taxes private equity and venture capital income at a lower, 15% rate, as compared to 35% on ordinary income.

As we recently discussed, it creates a dynamic that Romney would prefer to downplay:

1. Mitt Romney is worth $250 million.

2. He got rich by laying off American workers.

3. He pays a lower tax rate than you and the rest of the middle class.

4. He wants to be president so he can keep it this way.

If last night was any indication, it’s a subject that makes Romney uncomfortable, but it’s not going away.

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.