Step by step, the United States Congress is moving toward the adoption of legislation that would give the President authority to establish full diplomatic relations with the Vatican.
A House-Senate conference committee last week sustained an amendment to the State Department authorization bill that would repeal an 1867 law barring federal funds for a diplomatic mission at the Holy See in Rome. The bill will soon go to the House and Senate for a vote and, congressional sources say, it is expected to pass.
Although President Reagan has not mentioned the legislation publicly, he has signaled his approval of it. Sen. Richard G. Lugar (R) of Indiana, sponsor of the amendment, told the conference committee that he had personally spoken with the President and that Mr. Reagan not only approved of the amendment but ''welcomed'' it.
If the bill is adopted, the President is expected to follow through on the congressional recommendation and appoint a US ambassador to the Vatican. Administration officials point to the political benefits of such a move in an election year, when Reagan is wooing the votes of labor and Roman Catholics.
Similar US efforts to establish formal ties were made in the past but always blocked by public opposition. President Truman in 1951 was forced to withdraw the appointment of Mark Clark to the Vatican. A congressional move in 1977 also was defeated. In the absence of diplomatic relations, US presidents in recent times have simply maintained personal envoys to the Holy See.
Current efforts to repeal the 1867 law have received little public attention. Protestant and Jewish groups have protested the move but have not aroused much public interest, possibly because of the popularity of Pope John Paul II and the role he has played in the struggle for freedom in Poland. US lawmakers are also mindful of the repercussions in an election year of opposing the congressional move, say political observers.
As the amendment completes the legislative process, such groups as Americans United for Separation of Church and State (AU), National Council of Churches, and Baptist Joint Committee will continue to oppose it.
''The legislative battle isn't over,'' said a spokesman for AU. ''But even if it is passed and if the President nominates an ambassador, we would go to court. This is an important issue of principle - the United States government would be establishing relations with a religious faith. It goes to the heart of the First Amendment separation of church and state.''
Some 100 countries now have diplomatic relations with the Vatican. The US maintained formal ties only from 1848 to 1867.