At the request of Sens. Birch Bayh (D-Ind.) and George McGovern (D-S.D.) the U.S. Postal Service is investigating a recent mass mailing that attacked the two senators and three other Democrats as "baby killers."

The letter solicited contributions to a "$275,000 anti-abortion political war chest" for use against the five because they had voted for a woman's right, under certain circumstances, to terminate a pregnancy by abortion.

Besides Bayh and McGovern, the mailing called for the defeat in 1980 of Sens. Frank Church of Idaho, John C. Culver of Iowa and Rep. Robert F. Drinan of Massachusetts, a Roman Catholic priest.

Headed "Stop the Baby Killers" in large type, the letter was signed by Ohio State Sen. Donald E. Lukens, a former president of the Young Republican National Federation who served two terms in Congress in the 1960's. The letterhead bore the name of Rep. George Hansen (R-Idaho) as honorary chairman of the letterS sponsor. Hansen ran unsuccessfully against Church in 1968.

Lukens' letter was sent to 50,000 Catholics, Baptists and members of other faiths. In the text, the words "baby killers" and "murder" appear 41 times, along with grisly passages purporting to describe procedures.

Apparently, the spadework of the Postal Service's investigation has been completed.

"The whole matter has been laid in the hands of our general counsel at the national level," said Robert Fisher, postal inspector for the Washington division. "They are writing up a decision."

If postal authorities decide the letter is misleading, as Bayh, McGovern and Church claim, they can issue an administrative order that would prevent the further mailings Lukens is planning. But Lukens said he doubts that will happen.

"We would fight it every way we could," he said about such an order. "I doubt they will issue the order, because I don't see any blatant misrepresentation. They are treading on narrow ground, telling people what not to say.

"Basically, this letter represents my honest, sincere, hard-core feelings on this whole topic," he said.

The feelings of his targets are pretty basic, too.

"The way my record is characterized is scandalous and misleading," Bayh wrote to the Postal Service.

McGovern termed the mailing "an extreme example of misrepresentation of me, my political history, my personal beliefs and the issue in question."

And in Boise, George Klein, campaign coordinator for Church, called the mailing "slimy."

These people are part of the extreme right wing." Klein said. "Church opposes abortion except in cases where threr has been rape, incest, or danger to the mother's life, but these extremists only want straight line voting."

Even some anti-abortionists think the mailing went too far.

"The letter is stupid," said Paul Brown, head of the Life Amendment Political Action Committee. The committee is another anti abortion political group which has targeted the same five lawmakers for defeat, as have several other conservative political organizations.

"I didn't have anything to do with preparing the letter," Hansen said. "Mr. Lukens wrote it. I just lent my name. I probably wouldn't have expressed myself that way. When people get emotional they can get pretty graphic, and they can still be accurate."

According to Hansen, "Stop the Baby Killers" is a project of newly formed group called Americans For Life (AFL). He acknowledged that he is honorary chairman, but could not recall who recruited him for the post.

"Hansen's initials are on the letter," Lukens said when reached in Ohio. "These guys [the targeted members of Congress] are literal baby-killers. We haven't begun to mail. We are going to bomb 'em again next month.

Lukens said the AFL has only six members.

Jim Martin, a Virginia advertising man who handled the mailing, said it brought in contributions that exceeded costs by $2,000.A second mailing of about 60,000 is under way, said Martin, who learned the direct mail business from Richard Viguerie, a major conservative fund-raiser.

When the letter stirred negative editorials in Idaho, Martin said, "George Hansen called me. I told him the bottom line in my business is to raise money."

Martin said he also had a complaint from the attorney for the National Abortion Rights Action League (NARAL) because the mailing enclosed a facsimile of NARAL'S letterhead. The list of NARAL officials was overwritten with the words "baby killers" and "pro-death".

I said if you want to sue us, sue us." Martin said he recalled telling the attorney.

Nevertheless, Lukens said he had learned that the letterhead of an organization can't be reproduced without permission. So the second mailing won't include the NARAL letterhead.