Democratic spokesmen and low-income housing advocates said this week they are building roadblocks that they hope will derail the Reagan administrations' plans to pare $23 billion from low-income housing programs in l983.

Members of the House Subcommittee on Housing and Community Development intend to introduce within the next month an omnibus housing bill that would retain housing funds at 1982 levels, said Gerald R. McMurray, subcommittee staff director. The bill might include a bailout provision for mortgage firms, he said.

McMurray spoke Wednesday at the annual meeting of the National Low Income Housing Coalition, a lobbying group for the poor that is organizing grassroots opposition to Reagan's housing plan.

Speaking on behalf of subcommittee chairman Henry B. Gonzalez (D-Tex.), McMurray said that the budding Democratic alternative would reject administration proposals for deep cuts in the Department of Housing and Urban Development's housing subsidy and mortgage programs and in the Farmers Home Administration mortgage guaranty program. The subcommittee is part of the House Banking, Finance and Urban Affairs Committee chaired by Rep. Fernand J. St Germain (D-R.I.).

McMurray thinks opposition to the administrations' housing cuts is building in both the House and Senate. "I feel that the Congress is not going to go in the direction the President outlined in this '83 budget," he said

The Reagan administration wants to cancel about 300,000 of the 700,000 subsidized housing units now "in the pipeline," and to rescind about $16.4 billion appropriated for low-income housing in 1982 and l983, said Cushing N. Dolbeare, NLIHC president. She said the 1983 Reagan budget also would double or triple rental rates by forcing recipients of federal housing aid to count food stamps as income, and by requiring them to pay for all of their utility bills. On top of increased rents, some tenants would have to make up for a 15 percent cut proposed in Section 8 rental subsidies, she said.

Dolbeare also said that planned reductions in public housing operating subsidies could shut down some housing projects.

McMurray said Democratic subcommittee members want to block those changes with an alternative bill that would retain public housing programs and operating subsidies for federal housing programs at levels authorized last year by Congress. The alternative bill also will contain an as-yet-to-be determined bailout plan for the ailing mortgage-lending industry and will probably propose interest subsidies to aid home buyers.

It will also provide temporary assistance to home buyers who face foreclosure, possibly in the form of loans, McMurray said, noting the plan would not entail overall spending above 1982 levels.

McMurray predicted that public opinion will boost the more generous Democratic housing alternative.

"The potential is there for our bill; the potential is there for adoption and even for presidential approval," he told the group of housing advocates.

In any event, neither he nor Dolbeare expect that Reagan's recission plans will survive the l982 Congress.

Much more difficult, said Dolbeare, will be blocking Reagan's proposals that would raise rents for low-income families. To count food stamps as income, she said, would particularly affect the poorest--who receive the greatest number of food stamps. She said their monthly rents could go up by $50 or more per household as a result of the changes, and cited calculations by Mississippi housing advocates that indicate a mother on welfare with two children would have less than $20 left after paying her rent under the administration's program.

Dolbeare and McMurray see 1982 congressional elections as the key to enactment of alternative housing programs. "The last thing congressmen want to do is be blamed for some disastrous consequences of failure to pass a bill," McMurray said.

Coalition members said they plan an all-out campaign to get the votes of key members of House and Senate committees that deal with housing, appropriations and the budget. Behind them, the coalition members feel, will be grassroots support from more than 7 million residents of federally subsidized units, many of whom will feel the pinch of substantial rent increases if Reagan's plan survives.

"The only votes that are going to be changed this year are where local constituencies put pressure on local congressmen," said Kate Crawford, NLIHC's associate director. To that end she is developing computerized lists of constituents who can be mobilized to lobby key congressmen.

Most of the key congressman will be "fence-sitters," conservative southern Democrat "boll weevils" and moderate northeastern Republican "gypsy moths," according to Dolbeare. "We need to split off about 17 of them to carry what we want to carry in the House," she said.

Democrats also are hoping that their proposals to stimulate housing and to support thrift institutions will gain the support of powerful industry lobbies such as the National Association of Home Builders, the National Association of Realtors, building supply interests and influential savings and loan advocates, McMurray said.