The Reagan administration's renewed push to get a housing voucher program through Congress is running into stiff resistance from advocates of the poor and from Democrats on Capitol Hill.

Opponents charge that the proposal is not that different from part of the current Section 8 program, but to the extent that it is different, it would make it harder, rather than easier, for low-income people to rent decent housing.

When viewed in combination with other administration proposals redefining income and fair market rents, the plan is "nothing more than a veiled form of rent increase," said Cushing N. Dolbeare, of the National Low Income Housing Coalition.

"It's not as good a subsidy," said Roberta Youmans of the National Housing Law Project. "People would simply choose to go into horrible housing."

The voucher program, which Department of Housing and Urban Development officials call "the cornerstone of HUD's approach to helping poor people with their housing," is "a great theory," according to Rep. Bruce A. Morrison (D-Conn.), but the similar Section 8 program for existing housing "hasn't worked."

In broad outline, the voucher plan--which was proposed last year but did not pass--is fairly simple. Low-income tenants in effect would be given vouchers or certificates for the government's share of their rent. They could then shop for housing as they chose, using the voucher to pay part of the rent and their own money to pay the rest.

The administration sees several advantages to this. First, tenants would have a wider choice of housing. Because they could shop around, they could move to neighborhoods they liked better--with better schools, say, or better transportation. This freedom also would foster racial and economic integration, administration officials say.

A second advantage, according to the administration, is that the plan provides an incentive to cut costs. Because the government's share would be fixed--tied to the prevailing market rents in a given area--and the tenant's flexible, the tenant would have an incentive to shop for the best price.

The program will cause the tenants "to be a little more sensitive to what the costs are because there's a benefit to them," said W. Calvert Brand, general deputy assistant secretary for housing.

"People get the benefit of living where they want to live and having some real choice in both neighborhood and type of unit, and you still get a more-cost-effective type of program as far as the federal government is concerned," Brand said.

Critics of the program, however, argue that other Reagan proposals will stifle the benefits.

First, they note that the administration is seeking to count food stamps as income, which would lower a tenant's effective purchasing power.

Second, they say a proposal to base the fair market rent--which determines what HUD will pay in a given area--on the the 40th percentile of rents in older private housing will set subsidies below the level necessary to rent a decent place.

The critics say these conditions will mean that subsidized tenants will not be able to shop around and get decent housing.

"There's no marketplace," said Morrison. "These are people who are trapped, who have a desperately hard time finding an apartment that they can rent for a whole variety of reasons, and the notion that they're going to use their market power . . . , that's nonsense."

"Unless you have a very stringent standard as far as upkeep is concerned and a reasonable subsidy level in terms of making that affordable for a landlord, you're creating an inevitable situation in which . . . the unsubsidized person in a slum is replaced by a subsidized person in a slum which has had a cosmetic facelift."

Youmans also said she thinks that freedom for tenants to pay more for a unit would encourage landlords to press for higher rents. Under the current, capped system, they "sort of say, 'Why bother?' " and take HUD's fair market rent. Under the new plan, she said, they are allowed to ask more and will. "And I don't think many of the tenants have an extra $10."

The administration feels strongly that the program will work. HUD Secretary Samuel R. Pierce Jr. once told Morrison at a hearing that Morrison didn't know what he was talking about.

And, asked about possible subsidy of substandard housing, Brand said, "I'm really disappointed to hear that kind of comment. . . . For anybody to make that statement simply means that they haven't done their homework."