Republicans have a problem. Their base is killing them.

Mississippi Gov. Haley Barbour’s announcement Monday that he will not seek the presidency is just the latest sign that politically sentient Republicans fear their party’s voters have moved so deeply into la-la land that winning their support in next year’s primaries could render their nominee unelectable in November. “Friends of Barbour,” reports The Post’s Dan Balz, “said that he had come to the conclusion that Republicans can win only if they are totally focused on serious issues and not distracted by some side issues, such as Obama’s birthplace, that have arisen in the early going.”

But Republicans are massively distracted by birtherism. A New York Times-CBS News poll last week showed that while 57 percent of Americans believe that President Obama was born in the United States, against 25 percent who didn’t, just 33 percent of Republicans believed him American-born, while 45 percent did not. The Republican level of birtherism was effectively identical to that of self-identified Tea Party supporters, 34 percent of whom thought Obama was U.S.-born, while 45 percent did not.

Which is to say that the loopy, enraged divorce from reality of the Tea Potniks has infected the entire party.

That, indeed, has been the strategic premise of Donald Trump’s campaign, be it pseudo, proto- or provisional, for the Republican nomination. Nothing in Trump’s background suggests he actually believes the birther snake oil he is peddling with considerable success in GOP ranks. What his background, and foreground, do make clear is that he is utterly without shame. If stoking his campaign requires affirming the absurd beliefs of rubes whom he would instantly fire on his TV series, well — it’s worked, hasn’t it?

And it’s not just Trump. “Birther bills,” which require presidential candidates to produce their birth certificates, are moving through a number of Republican-controlled state legislatures. In Oklahoma, one such bill is expected to become law. In Arizona, the legislature passed such a bill, only to have Republican Gov. Jan Brewer veto it, calling it “a bridge too far.” Brewer didn’t specify where that bridge was headed, but surely she meant that an official Republican crossing-over into birtherism would place the party and its nominee on the paranoid fantasy side of the gap between the real and the imagined, while Democrats and independents gaped in amazement from the other side.

Brewer is not alone in her concerns. Karl Rove, still the GOP’s canniest strategist, told Fox News viewers that Trump was “off there in the nutty right” and a “joke candidate” for pandering to birther conspiracy theories.

The joke, however, may be on Rove and those reality-based Republicans trying to figure out a way to defeat Obama in the next election. If Rove really wanted to stop Trump and the birthers in their tracks, he should have looked Fox News viewers straight in the camera’s eye and told them to change the channel.

Widely shared paranoid fantasies existed long before Fox News and Rush Limbaugh, of course, but it’s increasingly clear that the success of news and opinion outlets devoted to counter-factual news in the service of partisan ends has driven a rift between their audience and the rest of the nation. If the rift were merely ideological, it wouldn’t pose a problem for the Republicans: Ideological rifts are the very stuff of politics.

Increasingly, though, the rift between the Tea Partyized Republicans and everyone else comes on the question of empiricism. Watch Fox News or listen to right-wing talk radio long and credulously enough, and you’ll end up believing that Americans found weapons of mass destruction in Iraq, that sharia law is being imposed on Dearborn, Mich., that climate change is a hoax and that Obama fears revealing the truth about his birth (a frequent theme of Fox’s Sean Hannity).

The authorities at Fox moved back from the brink a bit when they decided to let Glenn Beck go, but Beck was just one among many right-wing talksters whose cumulative effect has been to render rank-and-file Republicans a receptive audience for nonsense-spouting demagogues such as Trump. If the espousal of birtherism truly becomes a necessity for winning the Republican presidential nomination, the right’s war on empiricism will have served not merely to build and mobilize a base, but also to isolate that base from the majority of Americans who still inhabit, at least most of the time, a reality-based universe. Winning the support of crazies, Haley Barbour may have concluded, is no way to win the White House.