The Republican self-deception that draws the most attention is the refusal to believe that Barack Obama is American-born.

But there are Republican doctrinal fantasies that may be more dangerous: the conviction that taxes can always go down, but never up, for example, and the gathering consensus among Republican leaders that human-caused climate change does not exist.

I’m not saying that Democrats’ answers to the budget or climate challenges are necessarily right. You can make a case for smaller government or argue that there’s no point in America curbing greenhouse gases if China won’t.

But it’s hard to debate blind faith. When President George W. Bush and Congress lowered taxes in 2001, the justification, unlikely as it seems today, was a budget surplus. When the surplus melted away, that didn’t affect the ideology. Surplus or deficit, peace or war, healthy growth or steep recession — anything is an argument for tax cuts.

You can get a taste of this illogical arithmetic in Rep. Paul Ryan’s budget. The Wisconsin Republican lays out some ideas worth discussing to control entitlement costs. But by refusing to acknowledge that revenue will ever have to rise, even as society ages, he ends up, as the Congressional Budget Office noted (though not in so many words), in fiscal never-never land.

In Ryan’s vision, all federal spending other than Medicare, Medicaid, Social Security and interest payments will decline from 12 percent of the national economy (GDP) in 2010 to 6 percent in 2022 to 3.5 percent in 2050.

“For comparison, spending in this category has exceeded 8 percent of GDP in every year since World War II,” the CBO said. “The proposal does not specify the changes to government programs that might be made in order to produce that path.”

Of course not — because they are changes that few Americans would ever support.

The climate change denialism is a newer part of the catechism. Just a few years ago, leading Republicans — John McCain, Sarah Palin, Mitt Romney, Newt Gingrich and Tim Pawlenty among them — not only accepted global warming as real but supported some kind of market-based mechanism to raise the cost of burning fossil fuels.

Now polls show declining numbers of Republicans believing in climate change, and a minority of those believing humans are at fault, so the candidates are scrambling to disavow their past positions.

Palin, who as Alaska governor supported efforts to limit greenhouse gas emissions, in 2009 wrote in The Post, “But while we recognize the occurrence of these natural, cyclical environmental trends, we can’t say with assurance that man’s activities cause weather changes.”

Pawlenty similarly acknowledged on “Meet the Press” last year that “the climate is changing,” but added that “the more interesting question is how much of that is man-made versus natural causes.”

When I asked last week how Pawlenty would answer that “interesting question,” his spokesman responded by e-mail: “We don’t know [the] cause of climate change.”

Climate science is complex, and much remains to be learned. But if you asked 1,000 scientists, 998 of them would say that climate change is real and that human activity — the burning of oil, gas and coal — is a significant contributor. But Pawlenty’s supposed uncertainty is convenient, because if we don’t know the cause, then there’s little point in looking for a cure. And any cure is going to cost money, or votes, or both.

Democrats aren’t honest in these areas, either. President Obama does a good job of explaining how the Bush tax cuts helped cause today’s deficit, but then pretends that reinstating taxes on the rich alone can fix most of the problem. As the polls on climate change shift, he talks about green jobs and energy independence instead of global warming, as if there’s nothing out there but pain-free, win-win solutions.

To say that Republican irresponsibility makes it more difficult for Democrats to speak honestly is not an excuse. But it is a partial explanation. And while Obama may wish the climate change conversation would go away between now and 2012, he at least is not pretending the phenomenon is fiction.

Does Pawlenty believe what he says now? I’ve spoken with the former Minnesota governor. I know he is a smart man. As recently as 2008 he was supporting congressional action to limit greenhouse gas emissions. I do not believe that he believes those 998 scientists are wrong.

Which leads to another question: Should we feel better if a possible future president is not ignorant about the preeminent environmental danger facing our planet, but only calculating or cowardly?

To paraphrase Pawlenty, I don’t know the answer to that one.