''If it were voted down,'' says C. Fred Bergsten, ''it literally could unravel the whole international financial system. That is strong language,'' concedes the former US Treasury official, ''but I sincerely think it could happen.''
He is talking about a bill, already passed by the Senate, which currently divides the House, with Republicans and Democrats lining up on both sides of the issue.
Introduced by the White House and urgently backed by President Reagan, the measure would increase the United States contribution to the 146-member International Monetary Fund (IMF) by $8.4 billion. The existing US quota is about $13 billion.
Opponents denounce the bill as a bailout of spendthrift banks which, in the critics' view, have carelessly lent hundreds of billions of depositors' dollars to countries that cannot repay their debts.
Supporters of the measure, including the President and Treasury Secretary Donald T. Regan, claim that helping poor lands revive their economies is one of the best ways to create new American jobs.
Defeat of the bill might cause other nations to forgo their assigned IMF increases, leading to a general drying-up of loans to debt-ridden third-world countries. This in turn could push some heavily burdened nations into default.
Nonetheless, said House Speaker Thomas P. O'Neill (D) of Massachusetts on Wednesday, ''the votes just aren't there'' to pass the IMF quota bill, despite intensive lobbying by Mr. Reagan and Secretary Regan.
Opponents of the IMF increase, including some key Republican lawmakers, claim that default by one or more debtor nations would not trigger an international monetary collapse, although a few overextended banks might go under.
''Forty percent of all US exports,'' says Dr. Bergsten, now director of the private Institute for International Economics in Washington, ''go to developing countries'' - the very nations now deeply in debt to US, European, and Japanese banks.
When debt-strapped Mexico was forced to sharply cut imports from the US last year, Bergsten says, ''200,000 American jobs were lost.''
For years, developing countries formed the fastest-growing market for US exports, buying more goods than Japan and Western Europe combined. Now the trend has reversed, as mounting debts force third-world nations to trim imports.
Millions of American jobs, experts agree, ride on the ability of developing countries to buy farm goods and manufactured goods from the US.
A more immediate problem is to help the most burdened nations pay at least the interest on their enormous debts. This, ironically, can be done only if US, European, and Japanese banks continue to pour fresh loans into the debtors' coffers.
Here the IMF, set up after World War II to help nations meet balance-of-payments problems, plays a critical role, because of what Bergsten calls its ''carrot and stick'' approach.
The carrot is the promise of more loans, if - a big if - the government of the debtor land agrees to economic austerity measures laid down by experts from the IMF. Such measures are designed to correct structural problems within the beleaguered nation's economy.
The IMF also acts as a catalyst by requiring that private banks, as part of a total loan package, chip in with fresh loan money. For every $1 of IMF money, commercial banks worldwide are expected to furnish roughly $4.
Mr. Regan finds it ''worrisome'' that smaller US and foreign banks are pulling in their horns, refusing to lend more money to nations teetering on the knife-edge of default.
Without continuing IMF involvement in the world debt crisis, private lending to poor nations might mostly evaporate, enlarging the prospect that one or more debtor nations could plunge into default.
Yet the agency has about run out of resources to lend. Early this year, member nations agreed to boost IMF resources by more than 47 percent - from roughly $66 billion to $98.5 billion - to cope with the worsening debt situation worldwide. The US share of this increase is $8.4 billion, the amount that President Reagan now urges Congress to authorize. This money, like all contributions to the IMF, would be a loan, repayable with interest.
If Congress votes down the increase, the US share of IMF resources would drop from 19 to 15 percent, causing the US to lose its cherished veto power over agency operations.
The $8.4 billion breaks down into two parts - $5.8 billion for the IMF itself , $2.6 billion to a hard-currency emergency reserve funded by the West's major financial powers.