IN THE MAELSTROM of tax increases proposed by Maryland Gov. Martin O’Malley (D), it’s easy to lose track of which are most critical. For a start, it’s useful to identify which needs have been most and longest neglected — and near the top of that list is transportation.

It’s been 20 years since Maryland raised its gasoline tax, the largest source of transportation funding. As construction costs have risen, the revenue it yields has plummeted in real terms.

At this point, Marylanders (like Virginians, who last saw a gas tax increase when Ronald Reagan was president) are not paying for the roads they’re using. Despite the opening last year of the Intercounty Connector, north of the Beltway, the state has not kept pace with the growth of traffic or development.

It would cost $12 billion to satisfy just the top transportation priority of each of Maryland’s 23 counties and the city of Baltimore — to say nothing of Maryland’s share of maintaining the Metro system, whose problems, many of them the result of funding shortfalls, are the bane of Washington-area commuters. As things stand now, the state has nothing like that kind of cash.

In keeping with the recommendations of a blue-ribbon commission, Mr. O’Malley proposes to increase the state’s gas tax substantially over the next three years. Rather than hiking the flat per-gallon charge, the governor urged a phased-in sales tax, rising to 6 percent by 2016, the same rate charged for goods. If gas prices stay where they are, the governor’s legislation would add about 21 cents a gallon by 2015.

It’s easy to predict that the idea will be unpopular. A poll published last weekend in The Post found about three-quarters of Marylanders opposed to higher taxes for gasoline. And the governor’s strategy (if there is one) of rolling out a gas tax a couple of weeks after proposing new taxes on income, water usage, cigars and Internet sales hardly seems like a recipe for legislative success.

Still, by aiming high — the 6 percent sales tax on gas would yield $615 million annually, more than the increase urged by the commission — Mr. O’Malley may be giving Democratic lawmakers cover to settle for a lesser amount. It is critical that he fight hard for what he has rightly identified as a critically underfunded area.

Opponents say the price of gas is already too high, that family budgets are tight, that people don’t want to pay more. Fine. But then where would they find the hundreds of millions of dollars annually that no one disputes is needed to build and maintain an adequate transportation network — one that would include a Purple Line linking Montgomery and Prince George’s counties, and the Red Line expansion of Baltimore’s transit system? If they don’t like a sales tax on gas, what do they prefer? And how far into the future would they wait to start addressing the problem?