After complaining for years about the hardball methods of anti-abortion groups, proponents of legalized abortion are borrowing such tactics.

They have launched massive direct-mail letter-writing campaigns, hired veteran lobbyists, beefed up organizing efforts at the grass-roots level, financed expensive advertising campaigns and increased donations to political candidates.

But nothing symbolizes the shift in tactics better than two bits of propaganda that appeared during the Senate debate on abortion last week. One was a button worn by lobbyists and directed at Sen. Jesse Helms (R-N.C.), the Senate's most vocal abortion opponent. It pictures a coat hanger surrounding the words: "Abort Jesse."

The other was an advertisement, appearing in New York and Washington newspapers, that showed a senator in bed with a man and a woman. "The decision to have a baby could soon be between you, your husband and your senator," warned the ad, paid for by local affiliates of the Planned Parenthood Federation of America. The shift is a dramatic one for the federation that for years had taken what William W. Hamilton Jr., director of its office here, calls a "high-road" approach to the abortion issue.

Although its research arm, the Alan Guttmacher Institute, issued well-regarded but rather dull information bulletins about legislation, the federation did not advertise or have a single registered Washington lobbyist.

The federation, Hamilton says, now has a Washington office staffed by veteran lobbyists, a list of 300,000 donors to call on for help in grass-roots lobbying and almost $1 million budgeted to keep any anti-abortion legislation from becoming law this year.

Helms has complained bitterly about the federation's advertising campaign. There is a certain irony in this. For years, proponents of legalized abortion have complained about ads paid for by anti-abortion groups dramatically picturing fetuses.

Helms was at the hub of controversy during the Senate's long-awaited debate on abortion and school prayer last week. The debate centered on an inconclusive series of confusing parliamentary maneuvers.

It ended Friday when Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) moved to end a filibuster by filing a petition to invoke cloture when the Senate returns from its Labor Day recess. A cloture motion may be approved only if 60 of the 100 senators vote for it.

Behind the rhetoric and parliamentary skulduggery lay the fundamental fact that anti-abortion forces, at least temporarily, did not have the votes or vehicle to win a fight on the Senate floor.

Part of this was due to a long and divisive battle within the anti-abortion movement over the best approach to the issue -- a constitutional amendment, sponsored by Sen. Orrin G. Hatch (R-Utah), a "super bill" sponsored by Helms declaring that human life begins at conception or a more modest proposal by Sen. Mark O. Hatfield (R-Ore.) that would permanently prohibit the use of federal funds for abortion.

This fight left the once politically potent anti-abortion movement divided and discouraged.

Helms, finding that neither his original bill nor Hatch's amendment had the necessary support, fashioned a new amendment to a debt-ceiling bill. It combined some of Hatfield's proposal with some of Helms' original bill and an amendment to bar the Supreme Court from reviewing state laws permitting prayer in public schools.

The amendment makes it a finding of Congress that the Supreme Court "erred" in its 1973 decision legalizing abortion and that "the life of each human being begins at conception." It proposes to ban permanently use of federal funds for abortions.

Helms was candid about his purpose. "We are looking at the arithmetic of votes in the Senate," he said at one point. "Inasmuch as this is the only shot we have at it, we had better take our best shot."

Proponents of legalized abortion maintain that their lobbying effort in recent months caught Helms by surprise. "I think Helms went to sleep on this, and we didn't," says Nanette Falkenberg, executive director of the National Abortion Rights Action League.

NARAL, Planned Parenthood, the Religious Coalition for Abortion Rights, which represents a number of religious groups, and the American Civil Liberties Union were the key groups in coalition. They combined Washington lobbying with some hardball tactics in the home states of senators.

Hatfield, for example, modified his anti-abortion proposal after NARAL sponsored a newspaper ad in Oregon, saying: "Sen. Hatfield: You've been so right. Now how can you be so wrong." NARAL sent out legislative alerts to its 175,000 members, asking them to contact legislators, and Planned Parenthood contacted its 300,000.

Some Senate offices reported that for the first time they had been contacted by more proponents of legalized abortion than opponents. Sen. David F. Durenberger (R-Minn.) received 973 letters from proponents and 258 from opponents last week. Sen. Daniel Patrick Moynihan (D-N.Y.) received 1,438 from proponents, 576 from opponents.

John P. Mackey, editor of Lifeletter, a respected anti-abortion newsletter, said yesterday that anti-abortion forces have redoubled their efforts and will not be caught napping for votes after Labor Day.

"I think you'll see our people really geared up over the next two weeks," he said. "This is the showdown we've been waiting for."