A gubernatorial debate between Republican Ed Gillespie, left, and Democratic Lt. Gov. Ralph Northam on Sept. 19 in McLean. (Bill O’Leary/The Washington Post)

ED GILLESPIE, Virginia’s Republican gubernatorial candidate, likes to suggest the state’s economy is in a tailspin. Yet he is proposing a tax plan predicated on the assumption that average household income in the state is set to soar by about 50 percent in the coming four years.

That’s just a taste of the illogic at the heart of Mr. Gillespie’s tax proposal. It would reduce income taxes by 10 percent over three years, costing the state more than $1.3 billion in annual revenue, which, by an act of magical thinking, the candidate says wouldn’t put a dent in spending on schools, public safety, roads, parks and mental-health care.

Unhappily for Virginia voters, Lt. Gov. Ralph Northam, the Democratic candidate, is touting a fiscal plan that’s hardly more tethered to earthly reality than Mr. Gillespie’s. Mr. Northam would tailor his giveaway more narrowly to the poor — specifically, he says he’d scrap the state’s 2.5 percent grocery tax for lower-income residents — but leaves unexplained which state-supported services would bear the brunt of the giveaway.

Of the two, Mr. Gillespie’s plan, which holds out the promise of a free lunch in the form of the largest tax cut in modern state history, is the more fanciful and the more insidious. Mr. Gillespie says it would trigger an economic boom and yield tens of thousands of new jobs. In fact, there is a recent and telling model of the effects of just such a policy: Kansas.

There, another Republican, Gov. Sam Brownback, promised that his own enormous income tax cut, also the largest in state history, would be “a shot of adrenaline in the heart of the Kansas economy.” In fact, it turned out to be ruinous, crippling state revenue, sapping the state’s economy and education spending, and sending investors fleeing. In June, even the Republicans who control Kansas’s legislature had had enough; they joined Democrats in rolling back the Brownback tax cuts.

Perhaps mindful of the Kansas lesson, and a similar one in Louisiana, Mr. Gillespie says his own cuts would be triggered only if revenue projections held their ground. That’s a huge asterisk, and one that he may not want voters to pay much attention to before Election Day.

It’s made more insulting by Mr. Gillespie’s pledge that “average” households would save nearly $1,300 by his plan. It turns out that the “average” household he has in mind makes about $135,000 annually — about $45,000 more than the state’s real average. Confronted with that ticklish fact, the Gillespie camp responds that average household income is projected to rise precipitously in the coming four years — an argument that also sits uncomfortably with the candidate’s contention that the state’s economy is in the doldrums.

The Gillespie plan is an eye-roll; the Northam plan is piffle. Each would be given short shrift by lawmakers in Richmond, who know better. Neither is worthy of voters’ serious regard.