Many of us in the econ biz were wondering how the new leaders of Congress would respond to the sharp increase in American economic growth that, we now know, began last spring. After years of insisting that President Obama is responsible for a weak economy, they couldn’t say the truth . that short-run economic performance has very little to do with who holds the White House. So what would they say?

Well, I didn’t see that one coming: They’re claiming credit. Never mind the fact that all of the good data refer to a period before the midterm elections. Mitch McConnell, the new Senate majority leader, says that he did it, that growth reflected “the expectation of a new Republican Congress.”

The response of the Democratic National Committee . “Hahahahahahaha” . seems appropriate. I mean, talk about voodoo economics: Mr. McConnell is claiming not just that he can create prosperity without, you know, actually passing any legislation, but that he can reach back in time and create prosperity before even taking power. But while Mr. McConnell’s self-aggrandizement is funny, it’s also scary, because it’s a symptom of his party’s epistemic closure. Republicans know many things that aren’t so, and no amount of contrary evidence will get them to change their minds.

At least Mr. McConnell didn’t do what many of his colleagues have done when faced with inconvenient facts: resort to conspiracy theories.

Consider, for example, how some Republicans dealt with good news about health reform. Before Obamacare went into effect, they overwhelmingly insisted that it would be a disaster, that more people would lose insurance than would gain it. They were, of course, delighted by the technical problems that initially crippled the program’s website. But those problems were fixed, and enrollment soared. Their response? “They are cooking the books,” declared Senator John Barrasso of Wyoming, who now leads the Senate Republican Policy Committee.

But that was then. At this point we have multiple independent confirmations . most recently from Gallup . that Obamacare has dramatically expanded insurance coverage. So what do they say now? The law “will collapse under its own weight,” says Representative Paul Ryan, the new chairman of the House Ways and Means Committee.

Speaking of Mr. Ryan: Almost four years have passed since he and many others in his party lambasted Ben Bernanke, then the chairman of the Federal Reserve, for policies that they claimed would lead to high inflation and “debase the dollar.” The inflation never materialized, and the dollar proceeded to strengthen, but Mr. Ryan gave no sign of having been chastened . and many conservatives, including favorite intellectuals like Niall Ferguson of Harvard, became “inflation truthers,” insisting that the government is hiding price rises.

Oh, and Europe . whose central bank, unlike the Fed under Bernanke’s leadership, took those inflation warnings to heart and raised interest rates in 2011 . is now experiencing outright deflation, with terrifying implications for its economic and political outlook.

Then there’s climate change. It appears that 2014 was the hottest year yet, which should close the door on silly claims that global warming has stopped. But it won’t matter to Senator James Inhofe, who now leads a crucial environmental committee and has long insisted that all the science in this field is a liberal hoax.

Now, everyone makes predictions that turn out to have been wrong; it’s a complicated world out there, and nobody’s perfect. The point, however, is that Congress is now controlled by men who never acknowledge error, let alone learn from their mistakes.

In some cases, they may not even know that they were wrong. After all, conservative news media are not exactly known for their balanced coverage; if your picture of how health reform is working is based on Fox News, you probably have a sense that it has been a vast disaster, even though the reality is one of success that has surprised even the law’s supporters.

The main point, however, is that we’re looking at a political subculture in which ideological tenets are simply not to be questioned, no matter what. Supply-side economics is valid no matter what actually happens to the economy, guaranteed health insurance must be a failure even if it’s working, and anyone who points out the troubling facts is ipso facto an enemy.

And we’re not talking about marginal figures. You sometimes hear claims that the old-fashioned Republican establishment is making a comeback, that Tea Party extremists are on the run and we can get back to bipartisan cooperation. But that is a fantasy. We can’t have meaningful cooperation when we can’t agree on reality, when even establishment figures in the Republican Party essentially believe that facts have a liberal bias.