The letter that many of you got from President Reagan last week, an appeal for $37.5 million to elect Republican congressmen this year, is part of the Republican Congressional Campaign Committee's effort to keep its direct-mail fund lists fresh. The committee has a mailing list of more than 1 million donors, which is good for an average of about $3 million gross ($2.5 million profit) per mailing, but any list has to be constantly refurbished because it suffers an annual attrition of about 25 percent due to death, change of address and other reasons.

The committee does a sample mailing from various lists. If the response equals or exceeds the cost of mailing, it sends out a mailing to a larger sample, and if that is equal, etc., it does it again and again. It has done about 1.5 million mailings so far this year.

Last year the committee picked up a half million new donors from mailings to 4.5 million people, according to finance director Wyatt Stewart. They probably will net $25 million to $30 million over the 1982 and 1984 elections.

By comparison, the Democratic National Committee executive committee was told Friday that it has a mailing list of 125,000, up from 25,000 a year ago, with a goal of 250,000 by the end of the year. New donors gave $1.55 million in 1981, old donors $684,000. They hope for $3 million from new donors this year, $3.8 million from the old.