Direct rent assistance to low-income families is likely to replace construction subsidy programs as the federal government's main method of helping the poor meet their housing needs, Housing and Urban Development Secretary Samuel Pierce said yesterday.

The rent subsidies would take the form of housing vouchers, similar to food stamps. Pierce said in a speech to the American Bar Association that he expects a switch to the voucher system "in the not-too-distant future" and that the vouchers will go only to the poorest families.

While not opposing the voucher approach itself, some groups representing users of low-income housing fear that it would mean a severe cutback in aid to the poor. Others, including the AFL-CIO, oppose the voucher approach because they believe construction of more rental housing is the key to providing shelter for low-income families.

The government's Section 8 construction program involves HUD payments to builders and developers of low-income housing. The cost of this program is "absolutely prohibitive," Pierce said.

HUD's fiscal 1983 request for subsidized housing, submitted to the Office of Management and Budget last month, includes only enough funding for 140,000 additional rent-assisted units, sources said. This would include funds for the new vouchers for about 80,000 low-income families, they added. Since the Section 8 program began in 1974, close to 2 million units have been built or rehabilitated for low-income housing.

Cushing N. Dolbeare, president of the National Low-Income Housing Coalition, said her group still is considering its position on vouchers but would want it to be done on an entitlement basis under which eligibility would be defined and all those eligible would get the aid. "No one in the administration is seriously entertaining an entitlements program," however, she added.

"I think we would say that vouchers by themselves are not the answer to all low-income-housing problems," and the group would want them coupled with some method for increasing the housing supply, Dolbeare said.

Pierce, however, said that shortages of rental housing are a local phenomenon and should be dealt with as such.

"There is no current nationwide shortage in the rental housing market," Pierce told the ABA group. He cited a recent HUD study showing serious shortages in some areas but surpluses in others. Where more rental housing is needed, it could be supplied through the use of community development block grants or other targeted grants, he indicated.

The HUD secretary also gave an optimistic prediction of the effect of the president's economic program on the housing market.

"I believe there will be a direct and positive impact on housing from the program . This impact should start next year and build over a period of time, with perhaps the greatest or most noticable impact evident around 1984," he said.