THE ENVELOPE was stamped "urgent." Edging toward breathlessness, the aide who burst into the room to deliver the message said that the senator from Arizona was waiting for an answer.

Sen. Howard M. Metzenbaum (D- Ohio) interrupted his train of thought and tore open the envelope. He read the hand-penned note and then shook his head.

"Tell him I'm not prepared to accept this," he said. "It's not what we agreed to. . . . It's too late in the session. Twenty minutes to 5 on a Monday of the second week of a lame-duck session. . . . I doubt it."

What was involved here was a deal on a bankruptcy bill. Metzenbaum wasn't dealing and the bill was as good as dead. No deal here. No deal there. Someone ought to say it right up front in plain terms: Thank God for Howard Metzenbaum.

He drives his Senate colleagues bonkers, he gives big business apoplexy and many cynics in the press view him with disdain, as a bit of an arrogant grandstander. But that aside, he's about the most important man on Capitol Hill these days. Nothing moves through the Senate without his say-so.

As Congress lopes toward the denouement of its post-election session, Metzenbaum is blocking passage of at least a dozen bills that could cost consumers or the Treasury some $10 billion. The costs of other measures can't be calculated -- they are open- ended privileges for special people.

"It may be arrogant to say I'm not going to let a bill pass," Metzenbaum remarks. "You're damned right there's an arrogance. But I'm too old to go along for the ride. . . . (He's 65.) The Senate would be a better place if the leadership didn't let all these special-interest bills see the light of day."

Metzenbaum has done this before in the waning days of other sessions, barring the Treasury door to the smugglers of high-powered legislation, who seem to do their best work in times of chaos and distraction. And having just been reelected by an easy margin, it's certain he'll be doing it again as long as greed stalks the marble hallways and as long as the Senate's unanimous-consent rules allow one member to bring the place to a halt.

The techniques are fairly simple. By placing "holds" on bills, by objecting to floor consideration of a measure, by filibustering or achieving the same effect by proposing dozens of amendments, one member can stop the action.

A Metzenbaum "hold" on a bill means, for example, that its sponsors are honor-bound to tell him when they intend to call it up for floor action. At that point, he -- or another recalcitrant -- may object to its consideration. If the objection is sidetracked, he can filibuster the measure and talk it into the ground. If the filibuster is broken, he can achieve the same end by amending the bill to death.

These tools become more powerful in the tumultuous final days of a session, when members are eager to get home, when attention spans falter, and when flotillas of lobbyists troll the chamber for votes. These are the times when Metzenbaum uses the system to his advantage best.

There's nothing particularly new about any of this. What is different, however, is that Metzenbaum and a few allies in the last six years have turned -- in the name of the consumer and sound legislation -- to the same techniques that civil-rights opponents and other critics of social legislation used so adroitly over the decades.

The key is maintaining a constant watch on the floor -- either by being there personally or by having an assistant there -- to make sure that no ringers suddenly are cleared for floor action. Metzenbaum staff aides are on permanent floor watch, ready to ring the alarm bells. Just as zealous as their boss about dark-of-night legislating, they take turns writing dozens of amendments that are kept in reserve to tack onto bills that slip through their net.

Metzenbaum's hit list this year includes bankruptcy-law amendments; a bailout for ailing timber companies; antitrust immunity for beer wholesalers, ocean-shipping interests and pro- football-team owners; the gift of a federal railroad to the state of Alaska; and royalty-free leasing of federal oil shale tracts.

There are other "cats and dogs," as Metzenbaum calls them, but they have one thing in common: They are special-interest measures whose sponsors want them passed with as little scrutiny as possible.

Earlier this year, as Western senators pressed for quick passage of a bill extending hefty federal irrigation subsidies to big farmers, Metzenbaum dug in his heels and threatened a filibuster unless changes were made. Eager to leave for election campaigning, they gave in to the Ohio senator.

Last year, when the Senate was about to grant independent oil producers a multibillion-dollar tax break by reducing their windfall-profits taxes, Metzenbaum filibustered. As a result, the final version cost the Treasury $13 billion instead of the $26 billion originally envisioned.

Two years ago, when a hell-bent-for-adjournment Congress was in a similar rush to crank out more special-interest legislation, Metzenbaum stopped tax breaks and other goodies that would have cost something on the order of $5 billion.

He got his first taste of this in 1977, when he and former Sen. James Abourezk (D-S.D.) teamed up on an unsuccessful campaign to stop the deregulation of natural gas. All it took to get incensed about that, he says, was to see "the ripoffs" that were being engineered over his protests in the Energy and Natural Resources Committee.

For his efforts, Metzenbaum gets occasional favorable notice in the press here and in Ohio. He hears occasional quiet praise from some colleagues, but they rarely join in his blitzkriegs. And not infrequently he is the target of senatorial bile and contumely. Sen. Ted Stevens (R- Alaska), for example, earlier this year publicly called him "a pain in the ass."

There's a big irony here. Metzenbaum, as a result of his sallies against special deals for American businesses, is often labeled an enemy of the venerated free-enterprise system. Fact is, the free-enterprise system helped him become an immensely wealthy lawyer-businessman back in Cleveland.

It's just that there is more than one way of defining the system.

"Lobbyists perceive me as antibusiness," he says, "but I'm one of the most pro-free-enterprise people in the U.S. Senate. The strength of our system comes from free competition, from providing the greatest opportunity for the most people. Yet so often in the Senate we are asked by special interests to close the doors of opportunity for the rest of the country."

Worth noting is that virtually every one of the bills Metzenbaum is stopping this month is an effort to shelter certain businesses from the rough and tumble of the real world or reward them for their own miscalculations.

"I find that those who claim to believe in free enterprise are the first to turn to us for protection," he continues. "Utilities come to us; savings and loans are here; the banks are always here asking us to play referee; steel and autos come for help; the doctors and dentists don't want competition to work."

Because so few of his colleagues are willing to run the same gauntlet of scorn, Metzenbaum often leaves the impression of being a righteous gadfly -- an impression that recently prompted one of his four daughters to suggest that he channel himself away from being an abominable no-man. But it's not quite that easy, he says.

"I didn't seek this role. Most of these things that upset me are low- profile issues that the American people don't know much about. If they did, this place would operate quite differently."

The problem comes as much from the favor-seekers as it does from the Senate itself. Metzenbaum, although he is extended every collegial courtesy by the chamber's leadership, still is critical of the way they run the place. "The Senate and the Congress would be better served if the leaders would just say they won't take up cats and dogs. If in these final days they just took up urgent issues like the highway tax or the MX missile," he says.

"If they slammed the door shut on the rest, people like myself could tend to other matters. There are some things I'm trying to work on, like dealing with our grain surplus and feeding hungry people, natural gas and so on. But I can't do that with these other distractions."