Passengers prepare to board subway trains at the Naval Yard-Ballpark Metro Station. (Pablo Martinez Monsivais/AP)

THE LAST time Virginia took a major step in the direction of preparing the state’s transportation network for 21st-century traffic and commerce, it took more than a quarter-century to get there — that’s how much time elapsed between a landmark funding package in 1986 and, after its revenue stream had dried up and the state’s roads had crumbled, the next one, in 2013. The bad news is the 2013 deal did nothing for Metro. The good news is that Virginia’s Republican-controlled legislature is now fixing that omission.

In a move few would have believed likely, or even possible, just six months ago, Virginia lawmakers this month approved a $154 million increase in the subsidy the state provides annually for Metro’s repairs, upgrades and other capital expenses. That stunning success comes with an oversized asterisk, however, namely: About two thirds of the “new” funds for Metro really aren’t new at all. Rather, they’re raided from other transportation projects in Northern Virginia, where local officials, who had expected additional dollars, not diverted ones, are now panicked.

Gov. Ralph Northam (D), who has recognized that a zero-sum game is not the best solution to Northern Virginia’s transportation problems, should do all he can to amend the bill to minimize damage to other transportation projects . Those range from road widenings and intersection upgrades on major regional arteries to transit projects such as bus lanes and new Metro station entrances, to smaller plans such as bike lanes and sidewalk construction. Across Fairfax, Arlington, Alexandria and other Northern Virginia localities, the landscape is dotted with projects large and small that will stall as a direct result of new Metro funding.

For that residents can blame Del. Tim Hugo, a Fairfax lawmaker who is one of the last remaining Republicans representing Northern Virginia. Mr. Hugo, an anti-tax hard-liner and his party’s caucus chairman in the House of Delegates, opposed Democratic efforts to generate more new revenue, from slightly higher taxes on hotel and motel bills and on real estate sales in Northern Virginia. Downstate House Republicans, who had no dog in the fight — the new taxes wouldn’t have affected their constituents — deferred to him.

Mr. Northam could simply sign the bill as is. But he also has the power to amend it — for instance, by proposing that some $50 million be generated from increasing taxes on real estate sales. The amended bill would then go back to the legislature, which would accept or reject it in a one-day session next month.

In the closely divided General Assembly, with Democrats united in favor of additional, genuinely new funding for Metro, the governor would need a single Republican vote to prevail with an amended bill in the Senate, and a couple of GOP votes in the House. He should make the case that Virginians help themselves by paying for the transportation projects they use and need. Having allowed much of the state’s transportation infrastructure to deteriorate through the 2000s, the state would be foolhardy to go down the same path again.