In an attempt to aid troubled savings institutions before their numbers are further decimated through forced mergers, Rep. Fernand J. St Germain (D-R.I.) yesterday proposed that the federal government issue capital notes to guarantee their net worth.

St Germain, who is chairman of the House Banking Committee, amended a previous bailout bill that would have required the Treasury to put up as much as $7.5 billion in cash. The amended bill calls for no cash outlay unless thrift institutions are liquidated. Yesterday, St Germain urged industry trade organizations "not to kid themselves" that the Reagan administration would increase its deficit by bailing out the thrift industry and to accept this barebones alternative to more costly or comprehensive plans. The guarantees, bringing savings institutions reserves up to the minimum 2 percent of deposits required by law, would be limited to $7.5 billion.

Representatives of mutual savings banks, large and minority savings and loans appeared to agree with reservations on this course as a first step, whereas the chairman of the U.S. League of Savings Associations, Roy G. Green, held out for mortgage warehousing and government subsidies to lower mortgage rates at the same time, a $10-billion-plus program. Green argued against the present policy of forced mergers, which not only shrinks the industry but also would, he estimated, cost the government between $20 billion and $45 billion in the next two years, or more than any proposed bailout.

The thrift industry lost $6.4 billion last year, and savers withdrew $25 billion more than they deposited. This year could be worse. Without government assistance and without a drop in interest rates, said Green, 1,100 savings and loans, or one third of the industry, will have insufficient reserves left by the end of this year and will be merged out of existence by federal regulators; in 1983, 700 more will follow.

The Reagan administration has repeatedly stated its opposition to bailing out the thrift industry. However, sources suggested the latest off-budget proposal might make it easier to reach an accommodation with Congress. It would in essence transfer the responsibility away from government agencies with limited resources--composed of premiums assessed on the industry plus a line of federal credit--and on to the federal government itself.

Yesterday the House passed a resolution by a vote of 382 to 7 reaffirming that deposits of $100,000 in federally insured depository institutions are "backed by the full faith and credit of the United States." Treasury Secretary Donald Regan is expected to make a similar statement next Monday on behalf of the administration.

Together the Federal Deposit Insurance Corp., the Federal Savings & Loan Insurance Corp. and the National Credit Union Share Insurance Fund have resources of $18.4 billion and a line of credit on the Treasury of $3.9 billion. Putting the full faith and credit of the U.S. government behind insured deposits increases its potential liability to something over $1 trillion. (The FDIC reports that banks hold $984.3 billion in insured funds. The FSLIC reports that federally insured S&Ls have deposits of $515.4 billion, but it is not known what percentage of these are in accounts of over $100,000 and therefore uninsured.)

The actual figure is academic because such a default is unthinkable. Nevertheless, a small number of congressmen voted against the measure as "a sham." A spokesman for Philip Crane (R-Ill.) remarked, "It isn't good to fool the people; the resolution gives them confidence their savings are worth something. Should such a failure occur , the government couldn't borrow that money; it would have to print it. It wouldn't be worth anything."