WHEN IS A TAX not a tax? You were going to say: when it's a revenue enhancement. But you're wrong. It's when it's a user fee, and that's when the tax is a federal gasoline tax, and when an increase in it is politically defensible, economically sensible and increasingly supported as a way to pay for popular, necessary road, bridge and transit projects. That is how the White House prefers to describe a 5-cents- a-gallon increase in this levy, a proposal that is winning key bipartisan support in Congress. The president the other night got a little closer to it, too.

Not only would the increase finance the rebuilding of this country's crumbling highways and bridges and aid mass transit construction, but it also would be a solid response to unemployment. Among those who endorse the proposal for these significant reasons is Senate Finance Committee Chairman Robert Dole, who says a gas-tax increase to finance a roads-and-bridges bill would be "the best possible jobs bill that could be devised. . . . These new jobs could become available beginning as soon as 90 days after enactment of the program."

Transportation Secretary Drew Lewis, long an advocate of such an increase, says it would create 170,000 jobs directly, plus another 150,000 related jobs. The Democratic leadership in the House also recognizes this appealing aspect of the plan, which could raise an estimated $5.5 billion a year for transportation. Besides, there is no question that highway and repairs are long overdue all around the country. Similarly, an increase in the tax is long overdue; the present 4-cents-a-gallon rate has not been changed in decades.

So call it what they will, the proposal represents a quick and logical way to pay for projects that cannot be avoided--and that become more costly by the day. For that matter, the tax could withstand inflation better if it were set at a percentage of the gallon price. But if this "user fee" isn't increased one manner or another soon, users and non-users alike will find themselves footing the bill in ways they may find far more distasteful.