To explain the impeachment inquiry, the GOP has promulgated a flurry of baseless conspiracy theories about the forces arrayed against President Trump. It has been surreal to hear some of the nation’s most influential elected officials spouting ideas basically concocted from thin air. The House Intelligence Committee’s ranking Republican, Devin Nunes (Calif.), denounces “Ukrainian election meddling in 2016 . . . aimed against the Trump campaign.” The Judiciary Committee’s Louie Gohmert (R-Tex.) claims that if Congress could only talk to a certain three Democratic officials, then “every fact of this investigation” would be unearthed. Conservatives online hold that former White House aide Fiona Hill was a mole planted by George Soros.

Of course, no one purveys these theories with as much verve as Trump. The president has suggested that the nation’s first African American president was born abroad (he wasn’t), that the father of Sen. Ted Cruz (R-Tex.) had a part in JFK’s assassination (he didn’t), that Ukraine rather than Russia was responsible for election interference (it wasn’t), that the Mueller investigation empaneled by his own administration was a Democratic plot against him, that the Clintons orchestrated the prison killing of accused sex trafficker Jeffrey Epstein (who committed suicide, according to an autopsy) and that the Obama administration spied on his campaign, a claim most recently debunked by the Justice Department’s inspector general.

It certainly appears, more than ever, that “the truth itself is on trial,” as Peter Baker recently wrote in the New York Times. Some conservatives say Trump, and Trumpism, represent a new frontier of conspiratorial politics — that the theories are unique to Trump’s sensibility and peculiar to his paranoid White House. “Our politics seems more vulnerable to conspiracy theories and outright fabrication,” former president George W. Bush said in a 2017 speech. The “main problem” behind the proliferation of political conspiracy theories “is a president who is also a self-invented fabulist,” argued Washington Post columnist Michael Gerson.

But in truth, the stew of “alternative facts” has been simmering for more than half a century on the right fringe. The GOP had endless opportunities across the decades to banish these theories — about scheming bureaucrats, Jews, the Federal Reserve, the United Nations — and its leaders often saw them as absurd. But they were also useful, helping to rally support from an aggrieved government-hating base. So the party’s mandarins allowed them to fester and grow until they spread from the toxic fringe to the mainstream, which they have finally overtaken. It’s no wonder someone who embraced those ideas would become successful; Trump validates notions that his voters have long believed, ideas that the party refused to condemn and failed to repudiate.

Trump, then, isn’t what caused these conspiracy theories. He’s what happens when nobody stands in their way.

The intellectual life of the American right since Sen. Joe McCarthy’s rise to prominence in 1950 can be seen partially as a series of flirtations with conspiracists and a dedicated reluctance to read fringe crackpots out of its ranks. Yes, William F. Buckley Jr.’s National Review attacked John Birch Society founder Robert Welch, who called Dwight Eisenhower a “dedicated, conscious agent of the communist conspiracy,” in an iconic moment of movement history.

But conservative leaders more typically embraced the passion and support of the Birchers at the grass roots. At its 1964 national convention, the GOP rejected a plank condemning the society, along with its conspiratorial beliefs. Birchers such as John Rousselot and John Schmitz served in Congress as Republicans in good standing, while conservative oilman and family patriarch Fred Koch was a founding member of the society. Birchers took active part in Republican election campaigns and ran right-wing bookstores packed with conspiratorial treatises such as John Stormer’s “None Dare Call It Treason” (about Soviet “infiltration of American government”) and Fred Schwarz’s “You Can Trust the Communists (to Be Communists).” They served as members of local school boards and state legislatures. Other Republicans rarely spoke up to debunk the Birch view that fluoride in drinking water was part of a communist plot to cause cancer or control Americans’ minds.

At times, the wild-eyed conspiracism came from the top. Vice President Spiro Agnew delivered speeches identifying a cabal of Washington- and New York-based power brokers who supposedly controlled the news media, whipping up what journalists at the time considered a frenzy of anti-Semitism on the far right. (In 1976, a few years after he’d been indicted on bribery and other charges, Agnew published a novel pushing the anti-Semitic trope that the media’s “Zionist lobby” was controlling America’s foreign policy in the Middle East.)

At other times, understanding that they had to court the conspiracy-minded activists of “the base,” the party’s leaders went out of their way to confer legitimacy on extremists. Take the GOP’s relationship with Christian Coalition leader Pat Robertson, who won four states in the 1988 Republican presidential primary contest. In 1991, Robertson published “The New World Order,” holding that scheming government pooh-bahs were working to set up “world government, a world police force, world courts, world banking and currency, and a world elite in charge of it all.” He even suggested that President George H.W. Bush was complicit in establishing this new world order. Still, Republicans wanted votes and donations from Robertson’s acolytes, so Newt Gingrich, Oliver North, William Bennett, Jack Kemp, Jesse Helms and Dinesh D’Souza all happily spoke at his “Road to Victory” conferences. Sen. Bob Dole, a pillar of the Republican establishment and at the time a presidential candidate, told Robertson’s followers in 1995 that “I am proud to stand up here and say that I’ve been awarded a 100 percent voting record in ’93 and 100 percent in 1994 and 100 percent in 1995” from the Christian Coalition.

During the Clinton years, the crackpot theorizing became harder and harder to distinguish from the Republican Party itself. Conservative billionaire Richard Mellon Scaife sent investigators to Arkansas to dig up dirt on the president’s past, and when they failed, they settled for salacious fabrications that Bill Clinton was part of a drug-smuggling ring. Scaife became what The Washington Post called “a guiding force” behind Gingrich’s Contract With America and a later generation of tea party activists. The Rev. Jerry Falwell, a leader of the religious right (his son is a big Trump supporter), started selling a $43 videotape that, among other charges, said Clinton was responsible for the deaths of “countless people.”

Of course, Republican leaders didn’t always or uniformly tolerate gonzo conspiracy theories. Presidential nominee John McCain, for instance, contradicted one of his supporters at a 2008 campaign event, telling her that Barack Obama was not, in fact, “an Arab.” Onetime senator and 1996 presidential hopeful Arlen Specter (Pa.) also stuck up for the truth when he assailed two leading GOP conspiracy theorists, Robertson and Pat Buchanan, as leaders of the “intolerant right.”

Yet over the past seven decades, such voices — pro-fact, pro-science and pro-reason — lost the internal debate over how to treat the tinfoil-hat folks. And such moments of truth-telling by Republicans at all levels have been fleeting and rare.

House Intelligence Committee chair, Adam Schiff (D-CA) looks on as U.S. Representative Devin Nunes (R-CA) speaks as David A. Holmes, Department of State political counselor for the United States Embassy in Kyiv, Ukraine and Dr. Fiona Hill, former National Security Council senior director for Europe and Russia appear before the House Intelligence Committee during an impeachment inquiry hearing at the Longworth House Office Building on Thursday November 21, 2019 in Washington, DC. (Matt McClain/The Washington Post)

While the GOP was actively courting allies who propagated conspiracy theories, the Democratic Party took much the opposite tack — mostly rejecting the outlandish left-wing theories that surfaced from time to time. Although some research has found a recent uptick in Democratic support for conspiracy theories (a jump caused by a lack of political power), elected Democrats have typically fenced their party off from wild-eyed claims promulgated on the left. They never embraced the theory that the 9/11 attacks were an “inside job” carried out by the Bush administration, for example. During the 2004 presidential primary race, even the most lefty antiwar candidates — Dennis Kucinich, Howard Dean, Carol Moseley Braun — largely eschewed conspiracy theories that President George W. Bush and Vice President Dick Cheney had gone to war for oil or to enrich their friends in the construction business. The left has its share of anti-vaxxers who believe that vaccines cause autism, yet Democratic officials have battled that lie (which Trump himself has promoted).

One reason is that Democrats have been reluctant to use anti-government theories to tear down the things they would prefer to burnish, such as government and international institutions that they believe work for social justice and economic progress. Republicans’ conspiracy theories about powerful secret actors are consistent with their small-government views; it is a shorter leap for conservatives to portray Washington as a swamp where individualism, justice and decency go to die.

Thanks to years of GOP tolerance for surreal thinking on the right, conspiracy theories have moved from the fringe to the center. One poll by Fairleigh Dickinson University found that 63 percent of registered voters subscribe to at least one such theory. Some social science research suggests that people who feel alienated from politics are more prone to thinking conspiratorially. Trump capitalized on that climate of suspicion.

It’s easy to see the line from older conspiracy theories to the fact-free claims by Trump supporters today. White House adviser Stephen Miller has drawn anti-immigration rhetoric from white-nationalist publications like the 1973 French novel “The Camp of the Saints,” which envisions hordes of immigrants obliterating Western civilization. Breitbart News has featured anti-immigrant, anti-gay and anti-black theories that could have been found in an earlier generation of far-right publications, including American Opinion and American Mercury.

Perhaps the biggest change of the Trump era is the approach of Republican leaders. In the past, they indulged these theories but never really espoused them. Today, elective officeholders embrace them openly, in the face of explicit counterevidence from the government they purport to run. National security officials have repeatedly stressed in recent weeks that Ukraine did not meddle in the 2016 election and that those who repeat the claim are advancing pro-Russian propaganda. Yet most Republican members of Congress now seem to believe this theory, pointing to a thin report in Politico that documented no top-down, coordinated interference. Trump’s racism (calling Mexican immigrants “rapists”; telling four congresswomen of color who are all U.S. citizens to “go back” to their “crime infested” home countries) and anti-Semitism (he implies Jews are loyal primarily to Israel and motivated mostly by money) align well with old allegations that Jewish bankers and assorted globalists have enriched themselves while inflicting economic havoc on “the people,” as his closing 2016 television ad suggested.

Polarization has also made mainstream American culture hospitable to ideas once relegated to the margins. Voters have grown more tolerant of political leaders’ failings as long as they adhere to the correct ideological positions, opening a space for someone like Trump to win the presidency and then govern with the support of a minority of the electorate. And grass-roots opinion has shifted in favor of conspiracy theories, a pattern that both follows the cues of political leaders and incentivizes those leaders’ abasements. For instance, Twitter equips Trump to instantaneously air any theory, no matter how nutty, to raucous and affirming plaudits. Fox News also capitalizes on this shift to amplify Trumpian claims (“Russia didn’t hack our democracy,” Fox host Tucker Carlson recently parroted). It’s no wonder that, as Gallup found last February, 30 percent of Republicans see Russia in a favorable light. One recent Post survey of attitudes toward 2016 election conspiracy theories found that 26 percent of Americans believed that CrowdStrike, a cybersecurity firm, invented a story of Russian election meddling in order to hide Ukraine’s efforts to help Hillary Clinton.

Ultimately, Trump was the logical consequence of a posture followed for decades at the top echelons of the conservative movement: The batty screeds are silly, but since they help us, we won’t work zealously to purge them. Trump’s conspiracy-based capture of the GOP has less to do with him and his perspective than with a party that sought and often won the support of people who believe those notions.

[CORRECTION: A previous version of this story misstated the year of a Fairleigh Dickinson University poll about conspiracy theories.]