GOV. MARTIN O’MALLEY (D) has advanced a proposal to nudge Marylanders toward paying for the transportation infrastructure that they’re using and they need to keep pace with growth and the demands of a modern economy. Predictably, since the governor’s plan rests largely on a new gasoline tax, it has met with stiff opposition from lawmakers and others stressed by rising prices at the pump. Rather than wilt, the governor should make his case more forcefully.

The price of gas has more than tripled in the last 20 years. In the same period, state fuel taxes in Maryland have stagnated at 23.5 cents per gallon. By failing to keep pace with inflation and growing fuel efficiency, the state’s gas-tax revenue has lost more than half its purchasing power, even as population growth and wear and tear have degraded the state’s roads, rails and bridges.

A blue-ribbon commission found that Maryland needs an additional $800 million annually to meet its basic transportation needs. But the recession has sapped revenues, leaving the state’s six-year transportation program depleted. With available funds barely covering upkeep, new projects will remain on the drawing boards. Those include the Purple Line in Montgomery and Prince George’s counties and the Red Line in Baltimore.

With no new plan, it will become harder to get from one place to another, and the state will lose a critical competitive advantage. Taking into account lost time and money, commuters in the Washington and Baltimore areas already face the second- and sixth-highest traffic costs in the nation, as measured in a national survey by the Texas Transportation Institute. That’s bad. Without upgrades to the state’s transportation infrastructure, it will get worse.

The governor’s plan, which would raise an additional $613 million annually, sounds ambitious; in fact, it only begins to play catch-up. In the first year it would add 6 cents to the price of a gallon at the pump, with identical increases over the following two years, for a total of an 18 cent-per-gallon hike by 2015. A two-car family might end up paying $300 or $400 more per year.

That’s a burden, which is why the governor has to continue making his case. He should reiterate and emphasize that his plan, unlike previous ones, would keep abreast of inflation. He should stress that the tax increase would be frozen if gas prices rise more than 15 percent in a year. He should illustrate the benefits of better transportation infrastructure.

It’s not enough for Mr. O’Malley to pitch his plan and then stand pat. Given recent increases in gas prices, the timing isn’t great. But no one can know whether it will be easier or harder to enact a plan in another year. The only certainty is that, without one, the state’s traffic will get worse.