WASHINGTON -- "Some people say I'm extreme," an Indiana tea party leader told The New York Times at the height of the movement's rebellion in 2010, "but they said the John Birch Society was extreme, too."

Uh-huh. The society, which still exists, enjoyed its heyday in the early 1960s and saw Communists everywhere. Robert Welch, its founder, even cast President Dwight Eisenhower as a "dedicated, conscious agent of the Communist conspiracy." The group was so far-out that the founder of modern conservatism, William F. Buckley Jr., published a 5,000-word excoriation in the National Review that excommunicated the Birchers from the responsible right.

And on Ike, Buckley's friend and ally Russell Kirk offered a priceless riposte: "Eisenhower is not a Communist. He's a golfer!"

The tea party loyalist's observation might bring a chuckle from those who still remember the old Birchers, but it was also telling. Why have our politics gone haywire, why have our political arguments turned so bitter, and why was Donald Trump able to win the Republican nomination and, eventually, the presidency?

A central reason has been the mainstreaming of a style of extremist conservative politics that for decades was regarded as unacceptable by most in the GOP.

The extremist approach is built on a belief in dreadful conspiracies and hidden motives. It indulges the wildest charges aimed at associating political foes with evil and subversive forces. What's striking about our current moment is that such groundless and reckless accusations have become a routine part of politics -- all the way to the top.

On Thursday night, President Trump sent out a typically outlandish tweet peddling deceit by way of promoting Republican Ed Gillespie against Democrat Ralph Northam in next month's election for governor of Virginia.

Trump wrote: "Ralph Northam, who is running for Governor of Virginia, is fighting for the violent MS-13 killer gangs & sanctuary cities. Vote Ed Gillespie!"

If that tweet sounds like desperation, that's because it is. Northam, Virginia's lieutenant governor, has been leading Gillespie by 4 to 6 points in most polls. The Democrat was ahead by a whopping 13 points in a Washington Post-Schar School poll released the morning of Trump's tweet.

On one theory, Trump is trying to rally his enthusiasts to Gillespie to help him cut his polling gap. But the Trump ploy could also backfire: Roughly six in 10 Virginia voters disapprove of Trump's presidential performance, and nearly twice as many likely voters (30 percent) say opposition to Trump rather than support for him (17 percent) motivates their choice in the governor's contest. Trump's intervention could just as easily energize the larger group of Virginians who dislike him.

Tossing out the outrageous absurdity that the moderate, mild-mannered Northam is "fighting for" a gang whose motto is "Kill, Rape, Control" should be disqualifying for any politician who makes it. The claim originated in Trump-like Gillespie advertising rooted in Olympian leaps of illogic and distortion. The ads were taken apart by, among others, FactCheck.org, Washington Post editorialists and Post blogger Greg Sargent.

Ah, you might say, campaigns are often dirty. But current forms of right-wing dirty politics reflect a reversion to the old extremism. It has become part and parcel of "normal" politics and justifies kooky pronouncements as expressions of patriotism. Ordinary political acts are painted as diabolical. Dark plots are invented out of whole cloth. They are first circulated on websites that traffic in angry wackiness, and are eventually echoed by elected officials.

Thus did Rep. Paul Gosar, R-Ariz., allege last week, as Vice News reported, that the white nationalist demonstration in Charlottesville was organized by an "Obama sympathizer." Gosar further suggested it was funded by the far right's favorite Master-of-All-Demonic-Things, billionaire George Soros. Crazy, yes, but also ugly.

If the Birchers saw "The Illuminati," a shadowy 18th-century clique, as lying behind progressive treason, the new far right uses "globalists" as an epithet that is less obscure and more user-friendly.

The old extreme right linked all manner of actions by its opponents to Communism. The new ultra-right regularly ties its foes (as the Trump-Gillespie calumny does) to crimes ascribed to immigrants, or to radical Islam.

An authentic conservative knows that extremism is the antithesis of a philosophy devoted to the preservation of free institutions. The extremists hated Eisenhower because he understood this.

Although our current commander in chief is also a golfer, he otherwise has little in common with our 34th president. Trump is urging the right down a path that leads to nothing but trouble -- for conservatism, but also for our country.

(c) 2017, Washington Post Writers Group