RICHMOND, FEB. 9 -- Virginia's trucking and building industries, led by a group of Northern Virginia contractors, began an all-out attack today on the growing legislative sentiment here for a proposal to force truckers to cover their loads.

Spokesmen for both sides of the covered-truck issue, a hardy perennial of legislatures in Virginia and Maryland, said public support for a tarpaulin regulation is so strong that neither camp could ensure the outcome of a key vote Thursday in the House Roads and Internal Navigation Committee, which has summarily killed such legislation in the past.

"I've had four people approach me in the last 24 hours to change my vote or have me take a walk" and not vote on the pending covered-truck bills, said Del. Robert E. Harris (R-Fairfax), a member of the road panel and cosponsor of a bill calling for covered truck loads on Virginia interstate routes.

Several delegates said the building trades and trucking industries, which are flexing their considerable political muscle over this year's covered-truck legislation, were clinging to a two-vote margin to kill the legislation in committee.

"It's a mighty close vote here because we've all heard from our constituents, and this thing basically is a constituents' bill," Harris said.

Legislators from the state's most densely populated areas, notably Fairfax County and Norfolk, carried the message of the commuter-voters in their districts to the House panel today, saying truck debris has damaged countless cars and in some cases has injured and killed motorists.

"This is not a four-wheeler versus an 18-wheeler issue," said Del. Leslie L. Byrne (D-Falls Church), a leading proponent of truck covers. The question, she contended, is "when will we be proactive? When will we be preventative, instead of passive, to really control this problem."

The Maryland legislature has also often debated and always defeated covered-truck legislation, although there is an ordinance in Montgomery County that requires covered loads on county roads.

Spokesmen for the American Automobile Association, which represents 230,000 car drivers in Northern Virginia, said motorists statewide filed 39,000 claims in 1985 for windshields shattered by debris, but state police cited only 757 truckers for spilling their loads, a disparity that AAA says could be corrected by a covered-truck law.

Opponents drew a far different picture, saying rocks and debris rarely fall off trucks and strike cars. "They're flying up off the road, not off of trucks," said Del. Watkins M. Abbitt Jr. (D-Appomattox). "Y'all are perpetrating a hoax on us," Abbitt told proponents.

Peter Easter, a lobbyist for the Heavy Construction Contractors of Northern Virginia who led the attack against Byrne's bill, said a truck-cover law would duplicate an existing state requirement that trucks be loaded so their contents are not "dropping, sifting, leaking or otherwise escaping."

The $50 to $2,000 cost of tarpaulins, coupled with the "just unbelievable" time needed to cover a truck, would also pose an economic hardship on the industry, Easter added. "The trucking business is a tough situation," Easter said.

Several covered-truck advocates said they might be able to squeeze a version of the bill out of committee by limiting it to sand and gravel trucks, an alternative that opponents seemed to reject out of hand.