House of Delegates Speaker William J. Howell, R-Stafford, right, speaks during a news conference dealing with Medicaid expansion as Sen. Jeffrey L. McWaters, left, looks on at the General Assembly Building in Richmond, Va., Monday, Feb. 24, 2014. (Bob Brown/AP)

ASSAILING THE conservative bona fides of Virginia House Speaker William J. Howell (R-Stafford) seems a losing proposition — or would have seemed so until recently. Apparently, though, no one’s ideological purity is beyond doubt in today’s GOP, where those who stray from the righteousness of antitax orthodoxy may wind up in the cross hairs of the defenders of the true faith, who lie in wait, ready to mount a primary challenge.

Mr. Howell, speaker since 2003, has guided the House of Delegates from one conservative milestone to another — tightening abortion restrictions and voter ID laws; promoting a constitutional amendment to forbid same-sex marriage; opposing most gun-control legislation. The American Conservative Union and the antiabortion Family Foundation have each given Mr. Howell a rating of 100 percent.

Yet Mr. Howell, who, after years of foot-dragging, helped broker a tax deal to keep Virginia’s roads and rails from crumbling, now finds himself under attack as an infidel. Having backed new revenue for transportation, Mr. Howell now stands accused of lacking sufficient conservative resolve.

His accuser is Susan Stimpson, the former chair of Stafford County’s Board of Supervisors; last week, she announced she would challenge Mr. Howell in the Republican primaries next year (assuming he runs for another term).

Ms. Stimpson is a paragon of the party’s free-lunch crowd, which insists that, perhaps through magical thinking, Virginians can somehow avoid paying for the roads, rails and bridges they use.

Enamored of sloganeering and slim analysis, such Republicans declare that the state has a spending problem, not a revenue problem — although they never seem to say which spending should be cut to yield the billions of dollars required to maintain and improve the transportation network and keep Virginia competitive.

Ms. Stimpson also suspects Mr. Howell of engaging in treachery designed to enable Gov. Terry McAuliffe (D) to expand Medicaid under Obamacare. Never mind that Mr. Howell’s position of expanding Medicaid ranges from hell-no to over-my-dead-body. Neither is good enough for Ms. Stimpson.

She may be inspired by the defeat of Eric Cantor, the second-ranking Republican in the House of Representatives before he was toppled by a right-wing challenger in last spring’s primaries. After all, if Mr. Cantor had an Achilles’ heel, then why not Mr. Howell?

Perhaps. But in pushing the GOP further to the right, ideological purists like Ms. Stimpson may do their party no favors. After all, far-right candidates lost every race for statewide office in Virginia last year. And this year, a more pragmatic GOP candidate for the U.S. Senate, Ed Gillespie, came within a whisker of beating the supposed invulnerable incumbent, Mark Warner. Surely there is a lesson in that for Virginia Republicans.