Almost every year for the past decade some legislator has proposed a bill that would force truckers to put covers on their loads, only to watch helplessly as the measure, sometimes before the session ended, unceremoniously was killed.

This year the bill's latest champion, State Sen. Howard A. Denis (R-Montgomery), thought he finally had the votes to get it passed.

Then at lunchtime today, when some of the bill's supporters were grabbing a quick sandwich in the next room, the remaining committee members quietly voted the measure down, 9 to 4. Ten of the 25 committee members were absent at the time.

When the supporters of the measure trickled back into the committee session after lunch, they were infuriated. Both Denis and several of the committee members backing the bill had believed that the vote would not come until late that afternoon and accused committee chairman Torrey Brown (D-Baltimore)-an open foe of the measure - of underhanded dealing.

Brown responded simply that he had announced to the full House that he would hold a voting session today, and that the committee had been meeting for 45 minutes before the vote on the covered trucks bill. The committee, he said is "likely to vote at any time."

Later, however, Brown agreed to support any move to reconsider today's vote.

That did little to mollify some of the measure's angry supporters."What you have just witnessed is a step backwards into the Dard Ages," said Del. Judith C. Toth, the only member of the Montgomery delegation present for the vote.

Toth's fellow Democrat, Bienen of Prince George's County, also was fuming. "In five years in the legislature i've never been played with like that," said Bienen, who had stepped out for lunch just before the vote.

Denis, who had been dogging the steps of the House committee members for days, carrying around a pencil and a list of uncommitted delegates, was the most disappointed. The timing of the vote, he maintained later, was "a trick."

Denis had even spent his night collaring delegates at Fran O'Brien's, the legislator's favorite watering hole, and pleading his cause. "I didn't have a drink, I didn't have a pretzel, I just lobbied," a downcast denis said yesterday.

A representative of the one group that once had an immense interest in the bill-the powerful truckers' lobby-said that this year the truckers had spent far less effort opposing the measure than in the past.

This year, the Maryland Motor Truck Association, the main trucking lobby, had been concentrating instead on pushing for bills that would increase the legal length of trucks and the amount of weight trucks can carry, according to the group's lobbyist, Walter Thompson.

"I wasn't even following it (the covered trucks bill)," Thompson said when told of today's vote.

About an hour before the covered truck bill was deafeated, the House of Delegates passed a bill on increasing truck length.

Denis was the latest in the series of four senators who have promoted truck cover legislation. One of the former sponsored the bill. Sen. Arthur Dorman (D-Prince George's) said Howard Johnson only had 10 ice cream flavors when he sponsored.

Earlier efforts to push the bill had met with uick defeat. This year, however, Denis had managed to shepherd the bill through the Senate and was convinced-before today' setback-that he had gathered up enough votes in the Environmental Matters committee to bring the bill out of the full House.