Washington — Opponents of abortion, who suffered a major setback in the Supreme Court this month, are bracing for an expected defeat next week in the US Senate. ''I think the lines are pretty well drawn,'' says Sen. Orrin G. Hatch (R) of Utah. He expects his proposed constitutional amendment to fall short of the required 67 votes when it reaches the Senate floor Monday or Tuesday.
But the anti-abortion leader says he wants the vote anyway, partly because the debate will give his side a sounding board. Calling the estimated 1.5 million to 2 million US abortions a year ''abominable,'' Senator Hatch charged in an interview that some are performed because of ''cosmetics'' or ''because it was a girl rather than a boy.''
''The debate has to take place because people just have to hear these facts, '' he said.
He argues that ''most constitutional amendments take more than one debate.'' And even if this one fails this time, at least he would have put his colleagues on record either for or against. ''It's going to tell the people out there who are the pro-abortion people in Congress and who are not,'' he says.
The Hatch amendment, as modified by Sen. Thomas F. Eagleton (D) of Missouri, states simply, ''A right to abortion is not secured by this Constitution.'' It would open the door for states and Congress to pass laws restricting abortion.
''What we want to do is allow this society to make these moral decisions rather than judges,'' says Senator Hatch, calling nonelected jurists unqualified to rule on the issue.
Senate majority leader Howard H. Baker Jr. (R) of Tennessee has long promised to give floor time to the Hatch amendment. The debate comes on the heels of a series of Supreme Court rulings striking down an array of state restrictions on abortion. The new decisions reinforce the original 1973 decision in which the court upheld a woman's right to choose abortion.
The high court's action has apparently given little aid to the drive for a constitutional amendment. ''We're going to beat it,'' predicts a lobbyist for the National Abortion Rights Action League, adding that politicians ''have to read the polls'' that indicate a majority of Americans would oppose a Hatch-type ban on abortion.
Sen. Bob Packwood (R) of Oregon, chief opponent of the Hatch amendment, has labeled the anti-abortion forces as ''panicked.'' He said recently, ''I think time is not on their side.''
Not only is the amendment in trouble in the Senate, but even the more reliably anti-abortion House has shown signs of reversal. A recent amendment to exclude abortion from health benefits to federal workers passed, but by a narrower margin than in previous years. Pro-choice forces are claiming a gain of 20 to 25 House votes since the last election.
The wild card in the Senate debate is Sen. Jesse Helms (R) of North Carolina, arch foe of abortion.
''I think it's fair to say that Helms has no enthusiasm for pursuing a constitutional amendment on the floor with a 50-50 split,'' a Helms aide says. An amendment to the Constitution requires a two-thirds majority.
The Helms strategy is to pass a statute, similar to one he pushed for last year, that would define human life as beginning at conception, and force the high court to reconsider its 1973 ruling. The aide says that the hope would be to ''get a couple of more appointments to the bench'' who would take a stand against abortion.
However, Senator Hatch maintains that even a human-life bill probably would fail in the current Congress.
The only such legislation that both sides agree could pass in the current Congress is a permanent law to ban federal funding of abortion. Congress now enacts such bans on a temporary basis.
The abortion debate will be the first major Senate action this year on the so-called ''social issues'' that occupied the upper chamber for several weeks during the last Congress. Still likely to come are debates over school busing for desegregation and prayer in schools, as well as proposals to reduce the authority of federal courts in these areas.