The U.S. Catholic bishops put a powerful new spin in national politics last week with the draft statement they issued condemning most uses of nuclear weapons.

Their statement was a reminder that the Roman Catholic vote may be as open to overtures from liberals as from conservatives.

The Catholic vote has been viewed at least since the later 1960s as being up for grabs, no longer anchored in the Democratic Party. A number of theorists have foreseen a shift among Catholics to the Republicans, one they observed that could help make the Republicans the majority party.

Several reasons were given: Catholics were moving up in income, moving out of blue-collar jobs, were perceived as a group to be conservative on social issues, and to rally round the flag.

Republicans have moved to capitalize on these supposed tendencies. They courted the Catholic vote, especially on such issues as abortion and tax credits or other public aid to private schools.

What the bishops are doing makes this courtship more complicated.

The reason is that conservatives tend also to be hard-liners on defense. The bishops in their draft statement -- which in final form is intended to be the basis of the American church's teachings on nuclear war -- rejected the hard line.

On this moral and political issue, they came out on the softer side.

"Liberal" and "conservative" are notoriously fuzzy terms in this country. But if conservatives can still go at Catholics on the "social issues," the liberals now also can appeal to them on the defense build-up, the arms race and war and peace.

Conservatives have made some inroads into the Catholic vote in recent elections, just as they have been successful among voters in general.

Catholic voters in the industrial cities of the East and upper Midwest for the most part have maintained their loyalty to the Democratic Party. But Catholics who have migrated to the Sun Belt -- where the church is experiencing its fastest growth -- and those who have moved out to the suburbs in the North have been viewed as ripe for the plucking by conservative politicians.

A drift of Catholics toward conservative candidates was reflected in the 1980 presidential election. A Washington Post-ABC poll indicated that the total Catholic vote was 42 percent for Democratic President Carter, 46 percent for Republican Ronald Reagan, and 9 percent for independent John B. Anderson. Southern Catholics voted 41 percent for Carter, 54 percent for Reagan and 4 percent for Anderson.

A CBS-New York Times poll reported that blue-collar Catholics voted 44 percent for Carter, 48 percent for Reagan and 7 percent for Anderson. White-collar Catholics voted 35 percent for Carter, 55 percent for Reagaon and 8 percent for Anderson, the poll indicated.

But the 50 million Catholics in this country -- nearly a fourth of the population -- do not vote in lock step any more than any other large group.

The bishops also are hard to characterize politically. While the hierarchy has been strongly against abortion and in favor of aid to private schools, over the years it also has been strongly in favor of civil rights and endorsed a variety of other liberal causes, ranging from the Panama Canal treaties through gun-control laws to national health insurance.

The Catholic Church in America has undergone a major revolution in the last 20 years. It was brought about in part by the Second Vatican Council, on whose documents the U.S. bishops take their stand in condemning nuclear war, and in part by American Catholicism's transition from an immigrant church to a native one.

As an immigrant church in this country, Catholicism refrained from criticism and encouraged an outpouring of the more obvious forms of patriotism.

Of the nearly 300 Catholic bishops in active service today, at least three-fourths have been named to their posts within the last 10 years. With few exceptions they are men who are imbued with the spirit of church renewal and the reforms of the Second Vatican Council, and are committed to the council's mandate to apply Christian teachings to the secular world.

It was out of that conviction that they embarked two years ago on their present project of developing a position on nuclear warfare.

The bishops' statement is not binding on church members, but faithful Catholics are expected to weigh it seriously.

How seriously the faithful will take it when the statement is completed is always a question. Critics point to the fact that four out of five American Catholics ignore an even stronger directive of their church, the one banning artificial contraception.

The Rev. Robert F. Drinan, who was ordered two years ago by Pope John Paul to step down as a Democratic congressman from Massachusetts, suggested that "with some exceptions, [the faithful] would be with the bishops on this," referring to the bishops' nuclear war statement.

Michael Novak of the American Enterprise Institute thought the reaction of his fellow lay Catholics would be mixed.

"I think they'll feel a little pride that the bishops are trying to make religion relevant," he said, but added, "The bishops have encouraged Catholics to question the government, and inevitably Catholics are also going to question the bishops.... The more [the bishops] leave behind the Gospel and the more private judgment they get involved in, the more they are just like every other citizen."