LET'S SEE -- when we last looked in on the serene administration of Maryland Gov. Harry Hughes, he was sounding a wise warning about the likely need for an increase in the gasoline tax. It was, he said, a "fairly definite certainty," which most people took to mean "believe it." But now he's saying he may not request one after all, that "we'll know in the next several weeks." What the governor will say then is anybody's guess, but he was right the first time -- and he should stop wavering. The original warning was an important cue for legilators to begin preparing reasonable approaches to taxes and spending that could be considered promptly when the General Assembly convenes in January.

When he orginally talked about the gas tax increase, Gov. Hughes cited lagging transportation revenues and soaring construction costs as reasons; for rasing the levy at the pumps. But the latest word from the state's department of transportation is that the road-building program can be jiggered around enough to delay some projects. Which ones? He couldn't "remember any of them specifically," and for his sake, that's probably just as well -- because each project is dear to the heart of someone in the legislature. It also happens that many of those lawmakers likely to protest the loudest about road-building delays already feel that rural roads have been shortchanged in favor of urban rapid transit.

Probably there are some road projects that can and should be put off or forgotten entirely. But Maryland's roads, bridges, ports and subway projects all await spending; the last thing Gov. Hughes and his constituents need is a revival of the old rural-vs.-urban legislative feud -- culminating in a grand nothing-for-anybody plan and an agonizing, half-baked effort to raise taxes in a political campaign year.

It is not a matter of socking it to motorists so hard that they head for the borders to escape the tax, as District drivers appear to be doing these days. But a reasonable increase in the Maryland gasoline tax in 1981 is in order, and Gov. Huges should be girding the legislature to face up to this unpleasant prospect and to act on a modest measure before an extreme is necessary.