OVER THE next 20 years or so, according to a report commissioned by Virginia’s legislature, the state will need about $2 billion in transportation funds to support Dulles International Airport and nearby growth corridors. It will need up to $20 billion to improve freight mobility statewide and perhaps $10 billion to expand and upgrade the port at Hampton Roads. It will need tens of billions of dollars more to repair pavement; fortify flimsy bridges; ensure the safety of transit systems; build new tunnels and bridges; install the latest technology; and — most mundane of all, if not to millions of daily commuters — come to grips with traffic.

If the numbers make your head swim, you’re not alone. Lawmakers in Richmond haven’t begun to figure out where the money will come from.

Determined, and in many cases sworn, never to raise taxes for any purpose, the Republicans who control the General Assembly are standing idly by as Virginia’s transportation infrastructure crumbles, along with its economic competitiveness. Conservatively, the state’s hard-core transportation needs will outstrip available funds by $1 billion to $2 billion annually for the near future. To address that need, lawmakers plan to do, well, nothing.

In the fiscal year that ended this summer, the state had about $2 billion to spend on transportation construction. Five years from now, projections show, scarcely half that amount will be available, and most of it will come from the federal government. By that time, the state will have barely enough of its own construction funds budgeted to trigger federal matching funds — assuming those survive the appropriations process in Congress.

At the same time, outlays for maintenance and operations are expected to climb steadily as the state is forced to spend more and more patching existing infrastructure.

There’s no mystery about how Virginia has reached this juncture. The state has not increased its gasoline tax, the main source of transportation funding, in a quarter century. At the same time, inflation and more fuel-efficient cars have sapped the revenue produced by the tax.

Despite regular attempts to raise the gas tax by Democrats and a few (mostly Northern Virginia) Republicans in Richmond, the legislature has refused. GOP lawmakers have suggested shifting money from other areas of the budget to transportation. But it’s impossible to do so without making sharp cuts to education, public safety or health programs.

Gov. Robert F. McDonnell (R) understands the state’s transportation funding crisis and acknowledges that the dollars he’s found so far — by accelerating borrowing and auditing and cutting overhead at the Virginia Department of Transportation — are stopgaps. But Mr. McDonnell has refused to press for a major new funding source. Until he does, he will be complicit in digging Virginia deeper into a hole that will undercut its prosperity and prospects for generations.