Four years ago it seemed as if the "pro-hoice" groups that favor the right to abortion had won their fight. The Supreme Court ruled that states may not constitutionally prohibit abortion in the first three months of pregnancy, as some states had.

Partly on the strength of that 1973 decision, the number of reported abortions in the United States rose. That number is now 1.1 million a year - one abortion for every three live births.

That abortion-birth ratio is the same as in Denmark, Finland and Sweden, higher than in England (where it is one abortion for every five live births) but lower than in Hungary (where it is one to two).

Yet many Americans are still uncomfortable with the idea of abortion. And this week - only four years after their victory - it appears that the pro-choice groups are about to lose some important ground.

Both the Supreme Court and the Senate are expected to make abortion decisions in the next several days.

The issue now is not what it was in 1973: whether state and local governments have the right to ban abortion entirely. It is whether the federal and state governments have the duty to pay for "elective" abortions under Medicaid, federal-state health care for the poor.

Last year Congress - the House enthusiastically, the Senate reluctantly - passed legislation barring the use of federal Medicaid money to pay for abortions except where the mother's life is in danger.

About a third of all U.S. abortions are financed - saying the poor should not be denied a vital service the better-off can buy by Medicaid, so pro-choice groups for themselves - quickly took the legislation to court and won a stay.

Now, however, that stays seems threatened.

The Supreme Court ruled last week that state and local governments are within their rights if they decline to use their Medicaid money to pay for abortions, as many do.

This week the court is expected to make a similar ruling in the case of the federal government, and vacate last year's lower-court stay.

That would let last year's anti-abortion law - affecting this fiscal year's Medicaid appropriation - take effect, and meanwhile Congress is at work on a successor to restrict use of Medicaid money next fiscal year, which starts Oct. 1.

The House passed its version a week ago, as a rider to the Health, Education and Welfare appropriations bill.

Today this HEW bill comes up in the Senate. And there is not much fight left among senators who last year resisted the anti-abortion rider. "How many times can you go through the grinder?" asked Gaylord Nelson (D-Wis.). "I see no way the House is going to cave."

Last week's Supreme Court decision has also prompted fresh interest in abortion at the state level. Sen. Bob Packwood (R'Ore.), a leading opponent of anti-abortion restrictions on Medicare money, predicts: "Now that they no longer have to provide state funds, you're going to have 50 right-to-life battles," one in every state, as anti-abortion groups press for restrictions on spending.

Last week, for one early example, two New York state senators rushed bills into a closing session to prohibit Medicaid abortions, and some Michigan legislators plan to do the same.

Why, with abortion now so common, are legislators still so inclined to vote against it? One obvious reasson is the size and intensity of the "pro-life" forces. They estimate their strength at 11 million across the country, and their members are disposed - or thought to be disposed - to vote against "pro-choice" politican because of his abortion stand alone.

"Pro-choice" groups also not that President Carter and HEW Secretary Joseph A. Califano Jr. are opposed to using Medicaid money for most abortions.

Pro-choice groups today have two hopes.

The first is to attach qualifying phrases to this year's congressional rider that will allow at least some Mediciad abortions - some phrase allowing, for example, abortions in any case of "medical necessity."

The second is that conservatives may come back to abortion as they realize the cost of letting more welfare babies be born.