Democrats on the House Banking Committee unveiled an $18 billion legislative aid package for housing yesterday, repudiating the severe budget cutbacks proposed by the Reagan administration..

The Democratic alternative includes a new low-income housing-construction plan, loans to help families facing mortgage foreclosures and restoration of funds for a number of programs the administation wants to cut severely or eliminate.

It includes maintaining current commitment levels for the Government National Mortgage Association and FHA, which insure home loans, primarily for first-time buyers.

The congressmen--led by Housing subcommittee Chairman Henry Gonzalez (D-Tex.)--blasted the administration's main housing proposal, to replace low-income housing-construction programs with grants or "vouchers" for poor families who then would find their own apartments, rather than living in designated projects.

The voucher system would make the poor compete with the well-to-do for housing and would not stimulate much-needed rental construction, the panel members argued.

"It's a Halloween policy, an incomes policy masquerading as a housing policy," said Rep. Charles E. Schumer (D-N.Y.) about the voucher plan. "It's a disgrace and a sham."

The package announced yesterday is being introduced by Banking Committee Chairman Fernand St Germain (D-R.I.), Gonzalez and most of the panel Democrats. Gonzalez said he hopes the package can be on the House floor by May.

While the panel members appeared confident of carrying their program through the Democratic-controlled House, it would be bound to have trouble in the Republican-controlled Senate, where the voucher idea has received a more favorable reception.

The main initiative in the Democratic alternative to the Reagan proposals is a new form of federal assistance for construction of low-income rental housing to replace the current, much-criticized Section 8 construction program.

The funds--in the form of grants or loans or interest-rate reductions--would go to targeted areas in dire need and would all be provided at the beginning of the project.

The administration has proposed ending all new federally subsidized low-income-housing construction and halting production of 700,000 previously authorized units.

Currently, the federal government subsidizes low-income-housing construction both by providing low-interest financing up front and by subsidizing the rents of low-income tenants in the building for 30 years.