With a few taps at the keyboard, Hugh Donaghue can find out who wields power in rural New York.

He can get a fix on the local politicians, police officials and key businessmen, determine who their friends are and search out those who appear willing to use their influence for the latest political cause.

"It's like a gigantic 3-by-5 card," said Donaghue, president of The Tyson Capitol Institute, a Washington lobbying firm. "But it takes up a lot less space."

Judging by the response of about 100 lobbyists who were introduced to information systems yesterday at a conference and trade show sponsored by the American League of Lobbyists, they are well aware that computers increase their clout.

For example, lobbyists can learn in seconds the religion of every member of Congress or the names of Michigan residents who favor legislation to reduce acid rain.

Many of those who attended the conference here suggested that the image of the lobbyist is changing; the brash wheeler-dealer is being replaced by a calculating computer technician.

"Over time," Donaghue said, "the day of the big-time lobbyist going one-on-one with a congressman is going by the boards."

Representatives of Below, Tobe & Associates Inc., for example, displayed copies of the mass mailings the company can produce using its computers. The company sends mailings to people it knows are interested in a particular cause and asks them to send an enclosed form letter to a key congressman or other official important to the issue.

The innovation is in these return letters, which are randomly printed in 28 different type styles on many different kinds of paper. The people who read the mail might never know they are receiving form letters, explained a company official.

Some lobbyists cautioned that computers are not a substitute for ideas and said lobbyists also must be aware that many clients don't put much faith in technology, said Howard Marlowe, president of Marlowe & Co.

"Clients don't like to see these gadgets, so we don't show them the computers," he said. "Their eyes glaze over when they see a computer print-out."

But for working behind the scenes, computers have become an indispensible part of the job. "Everyone now seems to have one kind of computer or another," said William Miller, president of Miller Legislative Services. "Just push a couple of buttons and you've sent a letter to your entire congressional district."