THREE YEARS AFTER taking office, Virginia Gov. Robert F. McDonnell (R) has finally proposed to do something about the state’s critical long-term shortfall in transportation funding. The plan is serious and, in some ways, bold; it is certainly better than continued inertia and inaction. At the same time, it is inadequate and unbalanced, reflecting the grip that anti-tax ideologues continue to exert over Mr. McDonnell and Republican Party stalwarts in Richmond.

First, the good news. Mr. McDonnell, now in his final year as governor, has at last acknowledged a fundamental truth: There is no way to maintain and improve the state’s crumbling transportation network, now set to run out of construction money by 2017, without higher taxes. That basic fact, in a state that has not raised any sustainable new funds for highways and roads in a quarter-century, has been rejected by the GOP for years. Now the governor has conceded the point.

He did so by calling for a 16 percent increase in the state sales tax on all goods, in place of the current per-gallon gasoline tax, which would be eliminated. By 2018 that would yield an additional $183 million annually in revenue due to rising prices and economic activity. Add to that Mr. McDonnell’s plan for an additional $127 million annually, mainly from higher vehicle registration fees, and there you have it: $310 million in new annual revenue proposed by a Republican governor.

We applaud him for challenging GOP orthodoxy. But we also note that it is a paltry sum when weighed against the need for new transportation dollars, generally put at $1 billion a year. And the new money would come only gradually, diminishing its value in current dollars.

Mr. McDonnell would get part of the rest of the way to the $1 billion level by raiding general-fund money — for schools, higher education, mental health, public safety and recreation — and shifting some $283 million a year to transportation. Democrats have always choked on that. But the governor’s reasonable idea is that they may be more inclined to accept it if it’s accompanied by the tax increase.

That’s fine. It’s also laudable that the proposal would yield an additional $300 million to help finish Metro’s Silver Line extension to Dulles International Airport. To date, the state’s contribution to that major infrastructure project has been paltry; this plan sensibly addresses that problem.

But in the end, the combination of new taxes and fund transfers, which together would yield a little less than $600 million a year — and only by 2018 — is too little, too late. Recognizing that, Mr. McDonnell threw a Hail Mary pass. He proposed to dedicate an additional $222 million annually to transportation from a windfall that would materialize if Congress passes a measure known as the Marketplace Equity Act, which would allow states to collect sales taxes from Internet purchases. That would indeed be nice, but counting on Congress for anything is wishful thinking.

It may be true, as the governor suggests, that his too-modest plan will ignite a political bloodbath in Richmond. Republicans will balk at the higher taxes; Democrats at the raid on existing revenue. Maybe his plan is the best that can be hoped for. But if that’s the case, Virginians will be shortchanged.