A major roadblock to passage of a compromise gasoline tax increase was eliminated in the Virginia General Assembly today when Sen. Edward E. Willey (D-Richmond) told Gov. Gerald L. Baliles he will support a stripped-down House version of his original proposal.

Willey's support is seen as a key to winning Senate agreement, but the measure first must survive stiff opposition in the House, where it is set for a final vote Thursday.

"It's still a crap shoot," said David K. McCloud, the governor's chief of staff. He said Willey met with Baliles for 25 minutes this morning and agreed to consider supporting the amended bill. And, after a half-hour meeting with McCloud this afternoon, Willey dropped his fight to retain his original proposal, which was passed overwhelmingly by the Senate.

"It takes a very big man to get over the disappointment of not getting exactly what he wants," McCloud said of Willey's decision.

McCloud said that while Willey's support "obviously helps beyond words, this is anything but a fait accompli. This is a very difficult piece of legislation to move."

The amended proposal, offered by Del. Warren G. Stambaugh (D-Arlington) at Baliles' urging, would generate about $50 million for highway planning by raising the price of gasoline by 1.6 cents a gallon, while retaining a subsidy for gasohol production.

It gives the governor authority to decide how to spend the money, a decision usually made by the highway board and its commissioner. Baliles has said he wants the money to accelerate road planning.

Willey's plan would have raised about $700 million, by also increasing titling and registration fees and by eliminating the gasohol subsidy.

Much of the opposition to the bill in the House arises from a belief that any change in highway funding should be put off until September, when the legislators will return for a special session, called by the governor, to deal exclusively with transportation problems.

A committee named by Baliles is charged with proposing new methods of financing transportation needs, and is likely to recommend that the legislature scrap the state's 50-year-old pay-as-you-go formula for raising money for roads.

Among proposals Baliles has asked the Commission on Transportation in the 21st Century to consider are issuing bonds, adding new toll roads, and establishing separate funds for highway maintenance and construction.

Under the existing formula, all gasoline tax revenues go into a single fund, where maintenance projects take precedence. The combination of decaying roads and inflation has left an ever-smaller amount for new projects.