If you want to know why the United States is in such big trouble, look at the findings of a new Gallup poll. The percentage of Republicans expressing a “great deal” or “quite a lot” of confidence in science has plummeted from 72 percent in 1975 to just 45 percent today. (By contrast, the number of Democrats with confidence in science has grown from 67 percent to 79 percent.)

This is not some fringe movement we are discussing. This is a party that until the last election controlled both the White House and the Senate and that could easily recapture Congress next year. Think about what it means that a majority of one of the two major parties has lost confidence in science — essentially the same as losing confidence in logical, fact-based decision-making. The alternative is to embrace superstition, misinformation and conspiracy-mongering — and that’s just what Republicans have done.

Roughly a quarter of Republicans endorse QAnon’s lunatic beliefs, a third say that coronavirus vaccines are definitely or probably being used by the government to implant microchips, and a majority back the “big lie” that Donald Trump won the 2020 election. Many on the right will believe anything, no matter how loony and illogical, that comports with their political proclivities.

The Republican rejection of science makes it extremely difficult, verging on impossible, to deal with two of the biggest crises we currently face: global warming and the coronavirus pandemic.

Evidence of global warming’s calamitous consequences is growing daily. As my Post colleagues noted on Saturday: “Massive floods deluged Central Europe, Nigeria, Uganda and India in recent days, killing hundreds. June’s scorching temperatures, followed by a fast-moving wildfire, erased a Canadian town. More than a million people are close to starvation amid Madagascar’s worst drought in decades. In Siberia, tens of thousands of square miles of forest are ablaze, potentially unleashing carbon stored in the frozen ground below.”

It’s not as if the United States is exempt from these global trends. We just had the hottest June on record. Two states — Oregon and Washington — suffered nearly 200 deaths in a recent heat wave. Drought and wildfires are spreading across the West, inflicting more human suffering and steep economic losses.

Perversely, even as the effects of global warming become clearer, the percentage of Republicans who think that global warming is a major problem has declined. Another recent Gallup poll found that only 29 percent of Republicans believe that the effects of global warming have already begun (down from 46 percent in 1997), and only 32 percent accept that global warming is caused by human activity (down from 52 percent in 2003). The Republicans are in full flight from rationality.

The right-wing view was pithily expressed by Sen. Ron Johnson (R-Wis.), an inexhaustible font of absurdity, who called climate change “bulls--t." When Republican leaders speak of climate change at all, it is usually to promote bogus fears that Democrats are going to take away their hamburgers.

Even the so-called moderate Republicans in the Senate who negotiated with President Biden over an infrastructure bill stripped most of the funding for clean energy, because presumably they don’t think that’s an investment we need. While Republicans fixate on the (admittedly significant) costs of cutting greenhouse gas emissions, they ignore the catastrophic costs of allowing global temperatures to continue rising unabated.

The GOP position on covid-19 is just as nonsensical — and dangerous. Since the start of the pandemic, Republicans have been playing down the threat (remember when they were saying that covid-19 was less dangerous than the flu?), opposing mask mandates and complaining of the impact of public health regulations on the economy. Now the miraculous development of vaccines allows us to safely end mask mandates and social distancing and revive the economy. Yet 31 percent of U.S. adults haven’t received a single vaccine shot — and much of the reason has to do with growing Republican animus against science.

The Kaiser Family Foundation found that that the average vaccination rate in counties that voted for Trump was nearly 12 percentage points lower than in Biden counties. Another Kaiser poll found that just 52 percent of Republicans have gotten at least one shot, compared with 86 percent of Democrats. No wonder covid-19 cases are surging in red states, with Florida, Texas and Missouri accounting for 40 percent of all new cases last week. Now the Centers for Disease Control and Prevention is recommending that people resume wearing masks indoors. Good going, unvaccinated people.

Some Republican leaders are now belatedly urging people to get vaccinated, but it’s too late. After more than a year of undermining and insulting public health authorities, Republicans have left their followers deaf to the dictates of science and medicine. See, for example, this video clip of Arkansans booing and shouting while Gov. Asa Hutchinson (R) and a state health official try to explain that the coronavirus vaccines don’t cause infertility.

How many more people have to die from global warming and covid-19 before Republicans realize the deadly consequences of their hostility to science? Alas, the GOP might have gone too far down the rabbit hole to return to the land of fact and reason.