NEW YORK -- Senior citizens support Mitt Romney over President Barack Obama by a margin of about 20 percent, according to a recent Rasmussen poll.

But that doesn't mean the GOP presidential nominee is returning the favor: When he made comments in May dismissing households that don't pay federal income tax, he also was talking about at least 20 million seniors, most of them on Social Security.

That's the number Obama's campaign tweeted Tuesday morning. But the Tax Policy Center researcher whose work gave birth to "the 47 percent" meme said there are actually more Americans over 65 who don't pay federal income taxes.

In a clandestinely recorded video first reported on Monday, Romney says that 47 percent of the electorate will vote for the president no matter what. They are people, in Romney's words, who "are dependent on government, who believe that, that they are victims, who believe that government has the responsibility to care for them." The Republican candidate made no mention of the wide variety of payroll, excise and sales taxes those same Americans pay at the state and federal level.

Roberton Williams, a senior fellow at the Tax Policy Center, estimated that 20.2 million elderly "tax units" are not paying federal income tax in 2012. That's almost 27 percent of the 75.6 million total tax units with no federal income tax liability.

A "tax unit," Williams explained via email, is defined as "individuals or couples who either file a tax return or would file one if they had enough income. Tax units often have multiple members. An elderly tax unit may be a single senior or a couple, so the count of elderly PEOPLE not paying income tax is surely larger than the number of elderly TAX UNITS."

In other words, there could be millions more elderly who are, in Romney's words, "dependent on government." In an interview with Fox News Tuesday, the candidate said that "a lot of seniors pay income tax, and a lot of seniors don't like the idea that Medicare got cut $716 billion. So we're getting great support." (PolitiFact has rated Romney's claim that the health care reform law cut Medicare as "mostly false").

The reason so many seniors don't pay federal income taxes, according to Rudolph Penner, a senior fellow at the Urban Institute who was director of the Congressional Budget Office in the mid-1980s, is that they've reached a favorable age in the tax system.

"Elderly couples get an extra standard deduction," Penner said. "That is a benefit to a very large number of seniors."

"The other thing is the lenient taxation of Social Security benefits," he continued. Those benefits are untaxed when the combined income is less than $25,000 for single seniors and $32,000 for senior couple.

"Most Social Security benefits go tax free," Penner said.

Penner believes seniors should get fewer tax breaks because they often wind up benefiting those who are already well off. But he said Romney's suggestion that letting people off the income tax hook makes them "dependent on government" does not really hold for seniors.

"I can be said that Medicare and Social Security probably do accelerate retirement," Penner said.

However, he noted, "generally, it's presumed that older people aren't that affected by differences in incentives to work, so if that's what you're worried about, that's not a huge worry."