President Reagan, attempting to rekindle support among frustrated conservatives, launched a lobbying effort yesterday on behalf of anti-abortion legislation being filibustered in the Senate and a balanced budget constitutional amendment that is stalled in the House.

White House spokesman Larry Speakes also said Reagan does not oppose a school prayer amendment pending in the Senate, even though the Justice Department has said it believes provisions of the legislation are unconstitutional.

Reagan announced that he would play a direct role in the abortion debate in a letter to wavering Republican senators, urging them to break a filibuster against anti-abortion legislation sponsored by Sen. Jesse Helms (R-N.C.), and support Helms' proposal.

"It is time to stand and be counted on this issue," the president said.

Many of Reagan's conservative supporters have been increasingly frustrated by the administration's emphasis on economic issues and lack of action on social issues such as abortion and school prayer and the lack of progress on the balanced budget amendment. They also have been angered by Reagan's recent $98.3 billion tax increase and the decision to reduce arms sales to Taiwan in the future.

Reagan has long opposed legalized abortion, but previously has avoided endorsing any specific anti-abortion legislation. This was both because of divisions among anti-abortion groups and a fear that it would distract from his efforts to push his economic program through Congress.

Reagan has also endorsed a constitutional amendment calling for a balanced budget, but previously has done little to break it loose from the House Judiciary subcommittee on constitutional rights, where it has been bottled up by hostile Democrats.

Yesterday, however, the White House made a well-orchestrated effort to make the president visible on the balanced budget issue. Reagan called in 20 House Democrats and Republicans, who had cosponsored the constitutional amendment but so far had not signed a discharge petition to force it out of the subcommittee.

The petition has 203 signatures, well short of the majority of 218 needed to bring the amendment directly to the House floor, and only two more than the 201 the amendment's supporters had three weeks ago when they announced an all-out drive to get the additional signatures.

Later, Reagan spoke in favor of the balanced budget amendment at the Madison Hotel to a conservative group called the American Lobby, an organization created by the administration to lobby for it.

Asked why the White House was taking a neutral position on school prayer, despite misgivings about its constitutionality expressed by the Justice Department, Speakes said, "Well, I think it would be worked out in the courts."

Democrats expressed skepticism about Reagan's burst of activity, however.

"My personal view is he Reagan doesn't have his heart in it," said Sen. Max Baucus (D-Mont.), a liberal who has been filibustering against Helms' anti-abortion legislation. "It is a token appeasement of the right wing."

Helms' amendment, attached to a "must pass" bill to raise the federal debt limit, would permanently prohibit the use of federal funds for abortions and abortion research or training. It includes two controversial "findings" of Congress, which do not have the force of law.

The findings declare the Supreme Court "erred" in its 1973 decision legalizing abortion and that human life begins at conception. A rider to the amendment would prohibit federal courts from overruling state laws that allow prayer in public schools.

The debt limit bill must pass Congress by Oct. 1, or the government will be without authority to borrow money to continue in operation.

A liberal filibuster against Helms' proposals continued yesterday as the Senate returned from a 12-day Labor Day recess.

The Senate is to vote today on a move to invoke cloture and cut short debate. This would take 60 votes and the first effort to limit debate is expected to fail. A second vote is likely on Monday.

Also pending is a constitutional amendment, sponsored by Sen. Orrin G. Hatch (R-Utah), that would permit Congress or the states to regulate or prohibit the right of a woman to end a pregnancy by abortion. Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) yesterday was attempting to schedule a vote on the constitutional amendment early next week.

The amendment requires a two-thirds vote for passage, and is expected to fail. Helms' proposal requires only a simple majority. If the filibuster is broken, lobbyists on both sides of the issue expect a close vote.

But liberals moved to set up a post-cloture filibuster by introducing 680 new amendments to the debt ceiling bill. The amendments would have to be debated and voted upon before the bill becomes law.

Reagan, in a letter sent to senators, called Helms' proposal a "moderate approach," although he noted he also supports the Hatch amendment.

"It is vitally important for the Congress to affirm, as this amendment does, the fundamental principle that all human life has intrinsic value," Reagan said. "We must never become a society in which an individual has the right to do away with inconvenient life."

This letter was sent to nine senators, including such conservative stalwarts as Sen. Barry Goldwater (R-Ariz.), Thad Cochran (R-Miss.) and John G. Tower (R-Tex.).

Different letters were sent to Baker and Helms. In the Baker letter, the president said he was "expanding my efforts to support the cloture petition" and the Helms proposals. He urged "fellow Republicans" to support both, saying abortion "is one of the most important issues of our time."