Corey Stewart. (Calla Kessler/AP)

THERE WAS no better assessment of Corey Stewart’s victory in the Virginia Republican primary for U.S. Senate than that grimly offered by one of the last Republicans elected to statewide office. “Every time I think things can’t get worse they do, and there is no end in sight,” former lieutenant governor Bill Bolling tweeted. The nomination of Mr. Stewart, who built his political career pandering to fear and prejudice, represents another sorry embarrassment for American politics; his promise of a “very vicious and ruthless” campaign means further degradation in the civic discourse.

Mr. Stewart, chairman of the Prince William Board of County Supervisors, on Tuesday edged out a candidate backed by the party establishment to challenge Sen. Tim Kaine (D) in November’s critical midterm elections. His upset win, along with the defeat of Republican Rep. Mark Sanford in South Carolina, underscores how candidates aligned with President Trump and who have adopted his combative style succeed with the Republican base.

No one is Trumpier than Mr. Stewart. “Titular head of the Trump movement in the commonwealth” is how former White House strategist Stephen K. Bannon described him. Mr. Stewart cut his political teeth by fomenting attacks on illegal immigrants, speaks contemptuously of “establishment pukes” and, in coming perilously close to winning last year’s nomination for governor, courted white supremacists by embracing emblems of the Confederacy. No surprise, then, that Mr. Trump on Wednesday morning laid out the welcome mat, tweeting, “Congratulations to Corey Stewart for his great victory” and declaring he has “a major chance” of beating Mr. Kaine, whom he called “a total stiff.”

Other national Republican leaders were more circumspect. “No idea who he is. Not at all,” said Sen. Tim Scott (S.C.). “I just don’t know anything about him,” said Senate Majority Whip John Cornyn (Tex.). The National Republican Senatorial Committee has no plans to endorse Mr. Stewart and no plans to spend any money on his campaign. No doubt it recognizes the steep odds he faces against the well-regarded Mr. Kaine in a state that has trended blue in recent years and that last elected a Republican to statewide office in 2009. Republicans also fear that Mr. Stewart’s candidacy will drive moderate voters and women from the party, imperiling GOP prospects in races not only in Virginia but also nationally.

They should be concerned about more than such electoral calculations. The GOP should worry, as Mr. Bolling did in lamenting Mr. Stewart’s victory, about what has happened to proud party principles. As he put it, “This is clearly not the Republican Party I once knew, loved and proudly served.”

Most important, Republicans need to recognize that the toxic brand of politics Mr. Stewart practices, in which compromise is seen as selling out, pandering to racism a valuable tactic and winning at all costs a virtue, is bad for the country.