Political scientists long assumed that U.S. institutions were more open and sturdier than those in other countries. One manifestation of that robustness, the theory went, was that U.S. politics appeared largely free from troubling symptoms like conspiracy thinking. Foreigners — particularly in less-developed countries — might attribute the actions of their leaders to shadowy forces like the CIA or Israel’s Mossad, but Americans knew better than to think that offstage actors could have such influence on them.

That belief is no longer tenable. Conspiracy thinking has been normalized in American politics in a way that almost nobody could have expected a year ago. Today, it is plausible to think that U.S. politics could soon resemble cultures that most Americans once regarded as conspiratorial or paranoid.

Supporters of both parties traveled different roads to arrive here. For Republicans, the key ingredients included a combination of partisan outrage at the Obama administration and distrust of the “mainstream media.” For Democrats, the transformation began with the revelations that the intelligence community believed that Russian agencies had waged information warfare against Hillary Clinton’s campaign. Despite those differences, the net effect on how partisans view the other side has been chillingly similar.

As Richard Hofstadter argued in a 1964 essay on “The Paranoid Style in American Politics,” American politics haven’t always been entirely free of conspiracy thinking. Most of the time, though, such feverish imaginings have taken hold only among the extreme fringe, as with the anti-Communist extremist group the John Birch Society in the early 1960s. Rarely, toxic beliefs burst into the mainstream, as when the anti-immigrant Know-Nothing Party briefly became a mass movement in the 1850s or the heyday of McCarthyism. But greater openness, tolerance and education was supposed to have made those excesses a thing of the past.

Modern conservatism’s relationship with conspiracy theories has encompassed both models. In the 1990s, right-wing media outlets promoted conspiracy theories about the Clinton administration, but these were mostly fringe phenomena. The turning point toward mainstreaming conspiracy-mindedness came during President Obama’s era, when many on the right — most infamously, Donald Trump — embraced the gateway theory of birtherism.

Despite a complete absence of evidence in favor of the proposition and overwhelming evidence against it, the belief that President Obama’s birth certificate was fake took hold among a large segment of the population. By August 2016, 72 percent of registered Republican voters doubted that Obama had been born in the United States.

Mainstream Democrats, by contrast, prided themselves on being members of the “reality-based community” and gloated that (as Stephen Colbert put it) “reality has a well-known liberal bias.” The shock of defeat in 2016 changed that. Paradoxically, one factor pushing Democrats away from their faith in reality was their well-supported confidence in the evidence that a hidden actor — Russian intelligence agencies — had intervened in U.S. politics against them.

The U.S. intelligence community has publicly concluded that Russian President Vladimir Putin ordered an influence campaign against Hillary Clinton. Yet much of the evidence supporting that conclusion remains hidden from public view. That recalcitrance may be explained by intelligence agencies’ understandable desire to protect their sources, but it also leaves some gaps in the public story.

Consequently, the public’s willingness to accept the U.S. intelligence community’s conclusions is driven by a combination of trust in those agencies and, less respectably but crucially, the desire to accept those conclusions as true. In today’s polarized political environment, both factors follow party lines: Democrats believe the charges of Russian hacking, partly because they want to believe them. Republicans don’t, partly because they don’t want to.

In December, an NBC News-Wall Street Journal poll found that 86 percent of Democrats were worried by Russia’s interference in the election, while only 29 percent of Republicans were. Similarly, in January, a separate poll found that partisan views of the CIA were equally polarized, as 46 percent of Democrats, but only 29 percent of Republicans, approved of the agency.

The initial responses on social media to Tuesday’s publication by BuzzFeed of a document alleging that the Trump campaign had coordinated its actions with Russia (along with other, more salacious details) followed similar partisan lines. Left-leaning journalist Sarah Kendzior, for instance, viewed the document as confirming earlier rumors, while Fox News host Sean Hannity retweeted to his 2 million followers the WikiLeaks report that Trump lawyer Michael Cohen denied he had visited Prague (where the document alleged he had met with Kremlin officials).

Many Democrats seemed much more willing to entertain the report’s allegations than its relative lack of evidence would have justified. That may reflect, in part, confirmation bias driven by earlier acceptance of the intelligence community’s superficially similar report. Perhaps equally dismaying, Republicans seemed all too eager to accept on faith the Trump team’s even less-detailed denials. For many on both sides, motivated reasoning is beginning to eclipse reasonable standards of judging new claims.

It is difficult to be optimistic about the next steps in this path. The Trump team’s penchant for secrecy, brazen disregard for norms and hostility toward transparency has only fertilized the grounds in which paranoia will grow. For Democrats, the combination of truth and plausible-sounding theories about foreign involvement and Trump’s business interests, combined with the bitter taste of opposition, will lead into the temptation of steadily lowering standards for belief.

This is the same sort of stew that gives rise to political cultures dominated by rumor, innuendo and conspiracy thinking. Such brews are familiar to students of politics in regions such as Latin America and the Middle East. And we should remember that in many places, such conspiracy thinking reflects an exaggerated recognition that foreign powers, corrupt leaders  and hidden agendas did play an outsize role in such societies’ politics. Americans’ turn toward that direction may be, in that sense, nothing exceptional.

Eventually, polarization will ebb, and a shared reality will assert itself. Of course, the price of such an epistemic reset may be steep — a depression, war, Watergate. In the meantime, the consequences will be pain and strife. Until then, American politics will just be another example of the paranoid style.