Civil rights lawyers said yesterday they will fight in court a Reagan administration decision to stop denying tax-exempt status to schools that discriminate against blacks.

At the same time, Democratic leaders roundly denounced the Reagan administration policy shift and called for Congress block it.

The decision, announced Friday, "sets the country back many years," Rep. James R. Jones (D-Okla.), chairman of the House Budget Committee, said yesterday on the television show "Face the Nation" (CBS, WDVM). He said Congress should make sure the 12-year-old ban remains in force.

Meanwhile yesterday, Sen. Charles McC. Mathias (R-Md.) became the third senator to say he will propose a bill "to make the law crystal clear" against tax exemptions for private schools that practice race discrimination. Sens. Daniel P. Moynihan (D-N.Y.) and Gary Hart (D-Colo.) made similar statements Saturday.

Nonetheless, several civil rights lawyers said they felt the courts would be a more promising forum for battling the new policy.

"We've been notoriously unsuccessful in Congress making sure the existing ban is effectively enforced," said E. Richard Larson, an American Civil Liberties Union lawyer. "We've been out-lobbied by the Moral Majority types," he said, who have persuaded Congress over the years to attach riders on Treasury Department appropriations bills that kept the Internal Revenue Service from enforcing the ban.

Under a policy adopted by the Nixon administration in 1970, no private school or college was eligible for tax-exempt status unless it submitted a statement to the IRS that it did not discriminate on the basis of race.

Since then, some 100 schools have refused to submit such a statement, and have been denied tax-exempt status. Two, Bob Jones University in South Carolina and Goldsboro Christian Schools Inc. in North Carolina, challenged the ban in court.

Until Friday, the Reagan administration had fought them. But in a policy reversal, it announced Friday it would ask the Supreme Court to throw out the case and nullify lower court rulings affirming the ban.

Larson said that since the Jones-Goldsboro case is likely now to become moot, another judicical vehicle will have to be found to challenge the new policy.

The most likely, according to Norman Chachkin, of the Lawyers Committee for Civil Rights Under Law, is a still-active class action suit the committee brought in 1969 against the government on behalf of black school children in Mississippi.

The case, Green v. Connally, led the Nixon administration to impose the ban in 1970. Since then, however, the committee has been back in court on a number of occasions, arguing that the mere filing of a statement of non-discrimination does not guarantee that schools are, in fact, not discriminating.

It has asked for other criterion to be set. The matter is still pending before U.S. District Court here.