Senate candidate Corey A. Stewart (R) delivers a speech at the Trump Hotel during the Virginia Women for Trump's "Tea for Trump" in June in Washington. (Katherine Frey/The Washington Post)

Corey A. Stewart, the Virginia Republican provocateur whose political ascent has been built largely on his race-tinged appeals to white nationalists, wasted no time after becoming the party’s nominee for the U.S. Senate last month in blowing his dog whistle once again.

Mr. Stewart, who has led the Prince William County Board of Supervisors for more than a decade, has made his name in recent years by wrapping himself in the Confederate flag and defending Confederate statues (despite having grown up in Minnesota). Now, in furtherance of his message that the Confederacy was a noble cause, he has embraced the canard that slavery was not the impetus for the Civil War.

“I don’t believe that the Civil War was ultimately fought over the issue of slavery,” he said in a television interview last week, adding that most Confederate soldiers “didn’t fight to preserve the institution of slavery.”

Virtually every respected Civil War historian has debunked that myth, though it remains popular with apologists for the Confederacy. When, in 2010, then-Virginia Gov. Robert F. McDonnell (R) neatly omitted any mention of slavery in his proclamation of Confederate History Month, we asked James McPherson, dean of Civil War scholars, for his assessment.

I find it obnoxious, but it’s extremely typical,” he told us then. “The people that emphasize Confederate heritage and the legacy, and the importance of understanding Confederate history, want to deny that Confederate history was ultimately bound up with slavery. But that was the principal reason for secession — that an anti-slavery party was elected to the White House. . . . And without secession, there wouldn’t have been a war.” (Mr. McDonnell apologized and revised his proclamation.)

Virginia’s 500,000 slaves represented more than a quarter of the state’s antebellum population, and its economy, like that of much of the South, relied heavily on growing cotton, which in turn depended almost completely on slave labor. When Virginia adopted its Ordinance of Secession in April 1861, it did so in service to what it called “the oppression of the Southern slaveholding States.” Fifty of the state’s western counties then broke away, forming West Virginia, whose distinguishing characteristic at the time was a constitution that abolished slavery. When the Confederacy asserted its right to defend “property,” the property it had mainly in mind was slaves.

To airbrush slavery out of Civil War history is to minimize it. That’s obnoxious, as Mr. McPherson said, but no more obnoxious than Mr. Stewart’s previous acts of pandering, such as his assertion, after last summer’s violence in Charlottesville, that counterprotesters who stood up to neo-Nazis and white supremacists were to blame for “half the violence.”

A recent study found that U.S. students are largely ignorant about slavery and its central role in triggering the Civil War, and that high schools are doing a poor job of teaching that history. Mr. Stewart feeds on such ignorance and furthers it.