Key leaders of the Maryland House and Senate appear ready to scrap Gov. Harry Hughes' proposal that would add four cents a gallon to the gasoline tax in favor of a compromise plan that would add only about 2 cents to the average price of a gallon at the pump.

But even with general agreement among the various legislators interviewed, the compromise proposal could still be threatened by the kinds of special interest concerns and longstanding factionalism in the State House here that killed a similar gasoline tax proposal in the closing hours of last year's session.

Legislators working on the issue were cautious today about announcing prematurely any sort of a consensus for a compromise. But some key lawmakers, whose support is considered crucial for any gasoline tax to pass in this election year, agreed that the plan gaining the most momentum would replace the state's current nine-cents-a-gallon tax with a tax of 10 percent of the sales price.

That 10-percent plan would raise the price of gasoline immediately by about two cents a gallon. Future increases would be limited to one cent a gallon per year. As long as the price of gas continues to rise, motorists would be virtually guaranteed an additional penny-a-gallon tax every year. The tax would not be allowed to decrease, even if gas prices fell.

"There's a pretty strong feeling to just do away with the current tax and replace it with a 10 percent variable fuel tax," said Del. Thomas B. Kernan (D-Baltimore County), chairman of the joint House-Senate transportation oversight committee. Kernan said his committee will meet next week to hammer out the details of the tax proposal.

Hughes has proposed adding a 4 percent tax on top of the current nine-cent-a-gallon tax, for an increase of about four cents a gallon. Hughes' transportation specialist, Wayne A. McDaniel, said he was aware of the 10 percent proposal being discussed, but said he feared it would not raise sufficient cash quickly enough to replenish the state's dwindling transportation trust fund.

Hughes' plan would raise about $470 million for the fund in the first five years. The 10 percent plan would raise only about $400 million. "At this point, we're maintaining our position," McDaniel said.

Even with the compromise, most legislators said today that any sort of a gasoline tax will be difficult to pass this year in an election-minded General Assembly.

Also, some legislators voiced fears that all of the various transportation-related issues--including funding for mass transit, skyrocketing transit labor costs and making the Department of Transportation more accountable to the legislature--could upset the precarious compromise being formed on the gasoline tax rate.

"I would like to keep the . . . tax as separate as possible from all the other issues," said House Speaker Benjamin L. Cardin.

Adding other issues "would make a very precarious election-year bill fall short," said Del. Gerard F. Devlin (D-Prince George's). "You're talking about a bill that's only going to get about 75 votes as it is." Any bill needs 71 votes for passage in the House.

Hughes' recommendation to raise taxes on heavy trucks has received a much-needed boost when a panel studying the issue concluded that trucks are undertaxed for the amount of damage they do to state highways. That same University of Maryland study, when initially released last year, found that trucks paid more than their share, but the consultants reworked their calculations and came up with the new figures.