Consumer advocate Ralph Nader is criticizing the rise in postage rates and urging consumers to organize and insist on a voice in postal affairs.

The new postage rates, including a 3-cent rise in the cost of mailing a first-class letter, take effect Sunday. It will now cost 25 cents to mail a letter.Nader termed the increase a "harsh blow" on individuals and said the Postal Service routinely hears from businesses and non-profit groups about mail service and rates, but not from the average consumer.

Nader's solution is to form a group called Residential Postal Action.

"If we can get 10,000 people to write in, we could escalate this to something significant," he said in a recent telephone interview.

"Clearly, there is a great imbalance of representation," Nader said, and a consumer group "can help even the scales."

The rate hike comes at a time when congressional budget cuts have forced the Postal Service to curtail window hours and some collections.

At the same time, proposals to allow private firms to begin competing in the delivery of first-class mail have been put forth.

This combination of factors "threaten the stability of what was once a well-regarded public service" Nader said in a letter to Postmaster General Anthony Frank.

He called on interested individuals to write to Residential Postal Action, P.O. Box 19367, Washington, D.C., 20036.