SIX YEARS AGO, then-Virginia Gov. Timothy M. Kaine (D) jump-started the Metro system’s Silver Line extension to Dulles International Airport and points west, a massive project whose first phase is chugging toward an on-time, on-budget finish. Will Mr. Kaine’s successor, Robert F. McDonnell, now kill it halfway to completion?

That’s the question facing Mr. McDonnell (R) in the wake of his decision to deny critical funding to Dulles rail, one of the nation’s biggest infrastructure ventures. Even though it is vital to the Washington region’s economic vigor, to the airport’s success and to Virginia’s future, the state has all but turned its back on helping to pay for it.

Of the project’s total cost, now estimated at about $5.6 billion, Virginia’s contribution so far comes to a paltry $275 million — about 5 percent. Some of that was in the form of federal dollars for highway maintenance that the state simply repurposed, robbing Peter to pay Paul.

Mr. McDonnell has consented, grudgingly, to allocate another $150 million for the rail project. But after initially signaling that he would consider an additional $300 million, he apparently changed his mind in the face of opposition from downstate Republican lawmakers, to whom Northern Virginia and mass transit are terra incognita.

By denying the Silver Line the additional funding, Mr. McDonnell sharply increases the odds of two bad outcomes — and a third that would be even worse.

One is that commuters on the Dulles Toll Road, who are already on the hook for a lion’s share of the Silver Line’s overall cost, may face such severe toll increases that it forces many of them off the road and onto already clogged local arteries. Another is that the governor’s stinginess may prompt Loudoun County, one of the Silver Line’s funding partners, to withdraw from the project. Loudoun’s all-Republican Board of Supervisors is already wavering at the county’s $260 million commitment; if it pulls out before a July deadline, that would throw the project’s financing into disarray.

The real risk, particularly if tolls spiral out of control and Loudoun walks away, is that the second half of the Silver Line, from Reston to the airport, could unravel. That would be a staggering setback for the regional and state economies — and an enormous black mark on Mr. McDonnell’s legacy.

The underlying problem is that Mr. McDonnell’s transportation policy has been a failure. He campaigned on the promise of finding massive, ongoing funding for what he has frequently acknowledged is a system on the brink of crisis. Yet he has done little more than accelerate borrowing that Mr. Kaine already put in motion. That money will run out around the time Mr. McDonnell leaves office in 2014.

In the absence of reliable, ongoing new revenue — and yes, that means taxes — Virginia’s transportation network will continue to be chronically underfunded, and thus subject to repeated crises of the sort now threatening the Silver Line. And yet Mr. McDonnell, even as he recognizes the need for more revenue, will not challenge his fellow Republicans to seek it through taxes.

In their more partisan moments, some Virginia Republicans like to lash out at Mr. Kaine for his decision, in 2006, to hand control of the Silver Line project to the Metropolitan Washington Airport Authority, along with control of the toll road. It is Mr. Kaine’s fault, they say, that tolls may now become unaffordable for thousands of commuters.

But the fact is that the Silver Line was nothing more than a moribund plan on paper before Mr. Kaine’s move — which, by the way, was made in conjunction with key Republicans such as Rep. Frank Wolf (Va.) and then-Sen. John Warner. Today the Silver Line is nearly halfway to the finish line. It’s up to Mr. McDonnell to bring it home.