CONGRESSIONAL efforts are moving slowly to overturn a ruling by the United States Supreme Court last year that weakened the Constitution's protection of the free exercise of religion. But even proponents admit it won't be easy. The primary problem: The issue has become enmeshed in the bitter controversy over abortion.This past week Rep. Stephen Solarz (D) of New York and 41 other US House members introduced a bill, called the Religious Freedom Restoration Act, that would overturn that ruling. For decades government could not impinge the free exercise of religion unless it had a "compelling" reason. Then in a ruling a year ago the Supreme Court said the nation no longer could afford the "luxury" of such a strict standard of protection. The high court turned down the request of native Americans to continue to be allowed to use the drug peyote as part of religious ceremony. Later this year, probably in the fall, the House Subcommittee on Civil and Constitutional Rights expects to hold hearings on the Solarz measure, according to a subcommittee aide. Representative Solarz says his proposal "merely restores the balancing test" the way it had been before last year's court decision. The measure originally was to have been introduced in the House two months ago. What delayed it, say knowledgeable sources, was the effort of supporters to negotiate an agreement on the abortion controversy with representatives of the National Conference of Catholic Bishops and the National Right to Life Committee. That effort failed. These organizations say the bill might be used by pro-choice forces as the legal underpinning to assert a woman's right to abortion, if the Supreme Court overturns Roe v. Wade, which now permits abortion. They want an "abortion neutral" amendment added so that the measure could not be used in such a way. Solarz denies the bill is intended to aid the pro-choice position: "This legislation does not have behind it some kind of hidden, pro-abortion agenda." Without the support of most pro-life forces (a few back it now), the bill has slim chance of becoming law, say some backers privately. "I think it is going to be a struggle because the Catholic Conference is a formidable" adversary, concedes Forest Montgomery, counsel to the National Association of Evangelicals, which supports the measure. Mark Chopko, general counsel of the National Conference of Catholic Bishops, says his organization has "not taken a position" on the bill yet: "We hold open the possibility of opposing the Solarz bill, seeking an amendment" to it, or having a different bill introduced.