Rep. Fernand J. St Germain (D-R.I.) has warned the Reagan administration that its $8.4 billion proposal to increase funding for the International Monetary Fund is in jeopardy unless the Republican-controlled Senate passes a housing authorization bill that he wants.

St Germain, chairman of the House Banking, Finance and Urban Affairs Committee, delivered the warning "in the strongest possible terms" in a letter Tuesday to Treasury Secretary Donald T. Regan.

The administration, calling the IMF bill essential to the stability of the world banking system, narrowly prevailed in a 217-to-211 House vote earlier this month.

But many Democrats have criticized the bill as a bailout for large U.S. banks that have made huge loans to Brazil and other Third World nations that are billions of dollars in debt. The bill is awaiting a House-Senate conference.

Meanwhile, the White House opposes several parts of St Germain's $15.6 billion housing authorization bill, including provisions that would put the federal government back into subsidizing construction of housing for the poor and would increase federal rent assistance to some tenants in subsidized housing. Both provisions would reverse earlier legislation the White House pushed through Congress.

The House passed the housing bill after making major cuts, but a $17.5 billion version is stalled in the Senate.

"I have no doubt that the president of the United States could prevail to assure Senate passage of a housing authorization bill," St Germain wrote Regan. " . . . Unless a housing bill is enacted into law this year," he said, many House Democrats "will be constrained to vote against any conference report on IMF legislation."

A Treasury Department spokesman said that he could not comment on the letter because Regan had not yet seen it.

St Germain told the president in June that the housing bill is his top domestic priority and that Democrats would have trouble voting for increased foreign loans unless it was passed. He struck the same theme in his letter, saying that the administration is seeking votes for the IMF from Democrats who "have been continuously criticized for voting for spending measures."

An aide to St Germain said that the congressman was not threatening the administration, but merely explaining that the IMF bill "clearly is going to need all the help it can get."

St Germain's letter follows a news release by the National Republican Congressional Committee that attacked 20 Democrats for opposing an amendment that would bar the United States from supporting IMF loans to "communist dictatorships." The release said that the Democrats had "voted . . . to loan U.S. taxpayers' money to communist nations."

Democratic legislators quickly denounced the attack as "vicious, nasty, mean."

A spokesman for Sen. John G. Tower (R-Tex.), chairman of the Senate housing subcommittee, said that Tower has urged Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) to schedule a vote on the housing bill by early September.

The president already has signed the housing appropriations bill for fiscal 1984. But no new programs, such as the House provision to build 75,000 subsidized units, can take effect without an authorization measure.