U.S. churches, long in the forefront of lobbying to increase pressure on South Africa to dismantle apartheid, have begun a new push on Capitol Hill to enact stiff economic sanctions against Pretoria.

And they have found a strong ally - Sen. Paul Simon, D-Ill., who has returned to his Senate duties after an unsuccessful run for the Democratic presidential nomination.Simon held a late May hearing of his African Affairs Subcommittee at which top Roman Catholic, Protestant and Jewish leaders urged the United States to abandon President Reagan's policy of "constructive engagement" and impose stiffer sanctions.

Simon told the religious leaders that apartheid is not getting the attention it should in the United States, and churches and synagogues can help make it a "front burner" issue.

Church leaders agreed.

"Up until recently, I was reluctant to support sanctions," said the Rev. Herbert Chilstrom, presiding bishop of the of the Evangelical Lutheran Church in America. "But the South African people themselves are pleading with us for sanctions."

The church leaders all urged support for legislation sponsored by Simon, Sen. Edward Kennedy, D-Mass., and Rep. Ron Dellums, D-Calif., that would step up U.S. disinvestment in South Africa by ending new investments, loans, imports from and exports to South Africa, nuclear industry assistance, and end military and intelligence cooperation.

Joining for the first time in the call for stiff sanctions was the U.S. Catholic Conference, the social policy arm of the nation's Roman Catholic bishops.

In a statement to the subcommittee by Robert T. Hennemeyer, a former U.S. ambassador to Gambia and now foreign affairs adviser of the conference, Bishop Emerson Moore offered the bishops' "general support" to the comprehensive sanctions bill.

"In January of this year, the South African bishops reaffirmed their call for external economic pressures in the campaign to persuade the South African government to dismantle apartheid," Bishop Moore's statement said, noting the conference believes the sanctions bill is "a way to support the forces for peaceful change in South Africa and to make absolutely clear the church's abhorrence of apartheid."

Rabbi David Saperstein of the Union of American Hebrew Congregations told the Simon subcommittee that the apartheid system is anti-Semitic as well as anti-black.

Noting that the South African Jewish community numbers only 105,000, he said that there is an "active neo-Nazi in the right-wing Afrikaaner community. They are as ardent in their anti-Semitism as they are determined to preserve the racist character of the apartheid regime.

"As is often the case in history, racism and anti-Semitism go hand in hand," Rabbi Saperstein said.

While the religious leaders urged support for the sanctions legislation, they also noted that for the economic penalties to have any effect, other major traders with South Africa, such as Great Britain, West Germany and Japan, must join the United States.

Others testifying included the Rev. Avery Post, president of the United Church of Christ; the Rev. Edwin Mulder, general secretary of the Reformed Church in America; Suffragan Bishop Walter Dennis of the Episcopal diocese of New York; and the Rev. Charles Walker of the National Baptist Convention USA.