After many months of delay, Housing and Urban Development Secretary Samuel R. Pierce Jr. has passed the word that he is ready to give President Reagan a proposal to strengthen enforcement of the nation's fair-housing laws.

Pierce, whose department has been widely criticized for a passive approach to housing discrimination, had hoped to announce the legislative initiative with a bit of fanfare. But he was upstaged yesterday on Capitol Hill, where Sens. Charles McC. Mathias Jr. (R-Md.) and Edward M. Kennedy (D-Mass.) unveiled a much stronger fair-housing bill with 35 co-sponsors, a surprisingly high number that includes nine Republicans.

Under the Mathias-Kennedy bill, individual bias cases that HUD fails to settle voluntarily would be referred to administrative law judges, who could order corrective action and award legal fees and civil penalties up to $10,000. Under the current law, HUD can only refer the case to the Justice Department, which has filed only a handful of bias cases since Reagan took office.

Under the bill, real estate agents and landlords could appeal the HUD judge's decision to a three-member presidential panel or to a federal court. The bill also would broaden the law to ban housing discrimination against handicapped persons and families with children.

"Enforcement powers for housing discrimination complaints have been endorsed by every secretary of HUD," Mathias said. "Yet 15 years later we are without any enforcement teeth for Title VIII of the Civil Rights Act . . . . Evidence indicates that housing discrimination persists in our society in increasingly subtle forms."

Congressional sources say Pierce was asked to endorse the Mathias-Kennedy measure, but declined to take a position.

Civil rights supporters hope to avoid a replay of 1980, when Sen. Orrin G. Hatch (R-Utah) and other conservatives successfully filibustered a similar bill by arguing that it deprived defendants of their day in court.

Pierce, meanwhile, defended his voluntary approach to fair housing in a speech last week to the National Conference of Black Mayors in New Orleans. But he acknowledged that "we must have some other recourse in the event that conciliation fails," and he said his proposal would allow HUD "to go straight to court for equitable relief, or civil penalty, or both."

Further details were not available, and Pierce's plan still must clear a Cabinet council before it is presented to the president. AUCTION TIME . . . While Congress is debating whether to put the government back in the business of building subsidized housing, a conservative think tank has urged HUD to sell off some of the current stock of low-income projects.

The Heritage Foundation, in a report called "Agenda '83," says "the administration should press for the immediate elimination of operating and modernization subsidies for public housing," two programs that now cost $4 billion a year.

Contending that "low-income families are not as poorly housed as is widely believed," the foundation said: "Local housing authorities should be allowed to raise rents and to sell projects to the highest bidders." The proceeds should be used to fix up the remaining projects, the report says.

The foundation also took a swipe at one of Pierce's few urban initiatives, a proposed $150-million grant program to repair decaying rental housing. "It is not clear why the department has proposed a new rental rehabilitation program that will permit bureaucrats to decide which dwelling units are rehabilitated and to what extent," the report said.