President Reagan appealed to Congress yesterday to pass a constitutional amendment that would allow voluntary prayer in public schools and institutions, but he sidestepped another school prayer measure now before the Senate.

In his weekly radio address, Reagan declared that "the time has come for this Congress to give a majority of American families what they want for their children -- the firm assurance that children can hold voluntary prayers in their schools just as the Congress, itself, begins each of its daily sessions with an opening prayer."

The emotionally charged, five-minute speech stressed the importance of "spiritual faith" in the history of America, from the Pilgrims to Abraham Lincoln, with Reagan noting that "prayer is one of the few things in this world that hurts no one and sustains the spirit of millions."

Reagan used the occasion of Rosh Hashana, the start of the Jewish New Year, to deliver his punch line:

"Today on one of the holiest days of one of our great religious faiths, I urge the members of the Congress to set aside their differences and act on this simple, fair and long overdue measure to help make us 'one nation under God' again."

In rulings over the past 20 years, the Supreme Court has struck down school-prayer laws as interfering with the constitutional separation of church and state.

Reagan's offensive comes two weeks before the date Congress had hoped to adjourn and puts the Senate Republican leadership in a dilemma. A liberal filibuster against anti-abortion and school prayer legislation has tied up the body for three weeks.

Reagan noted that Congress had not voted on school prayer, but he did not mention the controversial school prayer legislation before the Senate. Sen. Jesse Helms (R-N.C.) has offered an amendment to the national debt ceiling bill that would strip the courts of the power to rule on school prayer issues.

Consideration of the bill was stopped by a liberal filibuster last week, and a cloture vote to halt the delay is scheduled tomorrow.

The administration position on the Helms amendment has been fuzzy, and the Justice Department has suggested that the proposed law may be unconstitutional. Asked yesterday whether Reagan supports Helms' effort, a White House spokesman said only, "We neither oppose nor support" the legislation.

Reagan said he had received assurances from Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) that "he'll do everything he can" to bring the president's preferred voluntary prayer amendment to a floor vote.

Reagan called on the House leadership to make an "equal effort," but prospects there appeared bleak.

An aide to House Minority Leader Robert H. Michel (R-Ill.) said that "Michel has the impression, given the fact that we don't control the House, that it is doubtful we will get to any deliberation of any of those social issues" this year.

Sen. Gary Hart (Colo.), responding to the radio speech for the Democrats, said Reagan was using the school prayer issue to avoid discussing the economy. Hart said the "president's economic program has failed, and apparently he'd rather not talk about it."

Hart said that the "issue isn't prayer or our belief in God. The issue is economic recovery and economic opportunity. That is our prayer."

Reagan announced his intention to propose a constitutional amendement on prayer in May. The amendment sent to Congress reads: "Nothing in this Constitution shall be construed to prohibit individual or group prayer in public schools or other public institutions. No person shall be required by the United States or by any state to participate in prayer."

Unlike the divisive abortion issue, Reagan's advisers view school prayer as a more politically popular tack.

A Washington Post poll last year found that while two-thirds of those polled opposed new anti-abortion legislation, the same number favored prayer in public schools.

Most main-line Protestant and Jewish groups, however, oppose a constitutional amendment.