Photo credit: Toni L. Sandys / The Washington Post

There was a lot of back-and-forth Sunday about Mitt Romney's position on insurance coverage for preexisting conditions.

It started with the Republican presidential candidate saying during an appearance on "Meet the Press" that he liked the Affordable Care Act's provision that requires insurers to cover preexisting conditions, and would support something similar. Hours later, his campaign clarified he did not, however, support a federal ban against denying coverage for preexisting conditions. Around 10 p.m., the Romney camp had circled back to the same position it held back in March: that the governor supports coverage for preexisting conditions for people who have had continuous coverage.

I wrote a bit yesterday about why this is different than ending preexisting conditions altogether. In short, it means that those who go a month or two without coverage could later be denied insurance for a medical condition. If, however, you have had a gap in coverage -- perhaps because you lost your job and couldn't afford it -- an insurer could not deny your application due to a preexisting condition.

A recent research project from the Commonwealth Fund explains why this distinction matters so much: There are tens of millions of Americans who lack continuous coverage.

Last month, the nonprofit organization released its annual look at gaps in health insurance. It found that, between 2004 and 2007, 89 million Americans had at least a single one-month gap in insurance coverage. They were not, in other words, continually insured.

That works out to 36 percent of the population between age 4 and 65. Some had longer gaps; about 12 million were uninsured for the entire four-year study period. A larger number - 14 million - experienced one single gap in coverage.

The continually insured -- the other two-thirds of the population -- already have a decently well-protected guarantee to coverage. HIPAA, a federal law passed in 1996, makes it difficult for employers to exclude a new worker's preexisting condition provided he or she did not have a gap in coverage. That is to say, much of what Romney intends to do here is already law. HIPAA also provides some guaranteed coverage for the continually insured in the individual market. (Jonanthan Cohn has a great piece on this if you want to know more).

That's how it works for those who never lose coverage. Then, there are the other 89 million. Some are young; some are old. Some didn't have coverage for four years; some only dropped for a month. But they all have something in common: They would be unlikely to be protected under the type of preexisting conditions ban that Romney has proposed.