Lawmakers entering the final stages of negotiations on sweeping housing legislation have encountered a new obstacle that could change the nature of the bills before a House-Senate conference committee.

Sen. William L. Armstrong (R-Colo.) is holding up an otherwise routine resolution that would extend the federal government's policy of preventing private landlords from opting out of a federal mortgage insurance program that forces them to keep rents low. Low-income housing advocates contend that if the policy is allowed to lapse at the end of the fiscal year Sunday, thousands of tenants will see their rents rise precipitously as building owners refinance the remainder of their 40-year mortgages.

The dispute corresponds with similar congressional struggles over the 1991 budget, and like that standoff, has been marked by partisan divisions, internal bickering and predictions of drastic consequences. Republicans on the House banking committee won approval for a plan that would offer owners incentives to keep their buildings under the federal program. Democrats on the Senate side, however, scarcely discussed the issue, deciding instead to pass a bill that continues the current practice of prohibiting owners from opting out of their contracts.

Barry Zigas, the director of the National Low-Income Housing Coalition, said such political differences cloud the fundamental agreement reached in both houses that a crisis is at hand that must be addressed.

"How many low-income tenants do we want to place on the table and see which one of us is right?" said Zigas, who predicts that 20,000 units -- or 60,000 tenants -- could be affected. "If this law is allowed to lapse, the only people placed at a competitive disadvantage are the tenants who could lose their homes."

Armstrong appears to be aligned with Republican House members who do not think the Senate has done enough to protect landlords who stand to lose money on buildings where rents are kept artificially low.

The makeup of the House-Senate conference that is expected to take up the issue next week "ended up stacked rather badly against the incentive approach," said Rep. Steve Bartlett (R-Tex.), a conferee. "There were no Democrats appointed on the House side who supported the approach the committee did."

"In my opinion, if preservation is not resolved in a way that protects and preserves property rights, there will be no housing bill," he said.

An Armstrong aide said he plans to hold the bill to assure that it is debated properly on the Senate floor.

Sen. Alan Cranston (D-Calif.), who chairs the Senate housing subcommittee, thinks Armstrong may not be able to win enough support from colleagues to stop the resolution, but acknowledged the Colorado senator could hamstring the process.

"When time is short, one person can create havoc," Cranston said.