I MUST admit I was as critical as most people when I heard that the Senate's new, ornate office building, originally budgeted for $45 million, would now cost an estimated $122 million by the time it was completed in 1981.

But then I talked to a lobbylist who persuaded me that the building was absolutely essential, no matter what it cost.

"It's impossible," he told me, "for lobbyists to do their work in the two Senate buildings now in existence. We have no place to sit while waiting to see senators. I've been in offices where there have been so many loobyists that half of us had to sit on the floor."

"What about the halls?" I said. "I thought that's where you people buttonholed most of the senators and their staffs."

"The halls used to be okay for lobbying," he replied. "But you don't realize how the lobbyist population has exploded in the last few years. We have lobbyists lobbying lobbyists. The halls of the two buildings are mob scenes. You'll start working on a senator and another lobbylist will suddenly elbow you out of the way and without so much as an "excuse me" start making his pitch for some other bill. Government can't operate under these conditions. That's why it's essential that we have this new Senate building no matter what it costs.

"No way. You have to be where the action is. When a bill is being debated on the floor you have to move in right away and warn the senator about the pitfalls in it. If you can't get to him immediately he might not realize how dangerous an amendments is to a lobby's cause."

"You're in a tough business," I said.

"Of course I'm in a tough business. But that's what I'm being paid for. At the same time we're being impeded at every turn. If it was just the lobbyists clogging up the halls I wouldn't mind. But now you have amateurs coming in every day wanting to see their senators, and the television and press people keep getting in your way. It's a jungle up there in those old Senate offices and the only thing that is going to save us is a new building."

"But everyone feels that with the cost of government skyrocketing the Senate should be more careful about overruns on its own offices. Some tax-payers worry that if the senators get more office space they'll hire larger staffs."

"Larger staff mean more efficient legislation. The extra space will make everybody's work easier. For example, it isn't generally known, but a great deal of the legislation in this country is originally drawn up by lobbyists. If we could have a place to work in a senator's office it would speed up things tremendously."

"Do you mean to say you're hoping to get desk space in the new Senate offive building?"

"Not really desk space. But it would be nice if a desk and typewriter were available so we could type an amendment as a bill is being proposed on the floor."

"I imagine the private restaurant on the top of the new Senate office building will be very helpful for you people, too," I said.

"That was my idea. I lobbied it through at the time," he said. "I pointed out that as long as they were spending so much money on a new office building, the least the senators could do would be to have a restaurant with a view. It's one thing to vote money for the departments of government, but it brings the reality much closer to home when you can actually see them."

"You make a strong case for spending so much of the taxpayers' money for another Senate Building."

"If I couldn't make a strong case," he replied, " I wouldn't be here."