But 40 percent of Americans are clueless about its sheer existence. Some think it’s been repealed by Congress. Some think it’s been overturned by the Supreme Court. A few probably think it’s been vaporized and replaced with a galactic edict beamed down from one of Saturn’s moons. With Americans you never know.

Image Frank Bruni Credit... Earl Wilson/The New York Times

According to a survey I stumbled across just weeks ago, 21 percent believe that a U.F.O. landed in Roswell, N.M., nearly seven decades ago and that the federal government hushed it up, while 14 percent believe in Bigfoot.

According to another survey, taken last year, about 65 percent of us can’t name a single Supreme Court justice. Not the chief one, John Roberts. Not the mute one, Clarence Thomas. Not even the mean one, Antonin Scalia. Though when it comes to Scalia, perhaps the body politic suffers less from ignorance than from repressed memory.

That we Americans are out to lunch isn’t news. But every once in a while a fresh factoid like the Obamacare ignorance comes along to remind us that we’re out to breakfast and dinner as well. And it adds an important, infrequently acknowledged bit of perspective to all the commentary, from us journalists and from political strategists alike, about how voters behave and whom they reward. We purport to interpret an informed, rational universe, because we’d undercut our own insights if we purported anything else.

But only limited sense can be made of what is often nonsensical, and the truth is that a great big chunk of the electorate is tuned out, zonked out or combing Roswell for alien remains. Polls over the last few years have variously shown that about 30 percent of us couldn’t name the vice president, about 35 percent couldn’t assign the proper century to the American Revolution and 6 percent couldn’t circle Independence Day on a calendar. I’m supposing that the 6 percent weren’t also given the holiday’s synonym, the Fourth of July. I’m an optimist through and through.