Reagan loses a tax bill, but gains aid plan for Caribbean

''We need tidbits of tax revenue from here and there to reduce the budget deficits. Now we've lost a piece of income that we needed.'' Thus a senior banking official described the decision of Congress to repeal a 1982 tax law requiring banks, corporations, and other financial institutions to withhold taxes on interest and dividends.

This was not a new tax, but simply a move, through mandatory withholding, to collect taxes owed by Americans who break the law by failing to report their full income from interest and dividends.

Over the next five years, according to US Treasury officials, the government would have garnered up to $13.4 billion in evaded taxes. Much of that income now will be lost.

Repeal is a defeat for President Reagan, who strongly backed the withholding measure as a way to crack down on tax evaders and shrink the budget deficit without asking for new taxes. Despite earlier threats to veto repeal, the President is expected to swallow hard and sign the bill, because Congress attached to it something Mr. Reagan wants even more - his Caribbean Basin Initiative.

Through this initiative, which will expand US economic aid and widen American markets to exports from Caribbean nations, Mr. Reagan hopes to accomplish two things:

* He seeks to dampen Marxist influence throughout the Caribbean region by helping to improve the economies of impoverished lands.

* The initiative stresses the economic side of the President's embattled Central American policy, whose military aspects are increasingly controversial.

''We have the Caribbean Basin Initiative,'' said a top administration official, when asked why White House threats to veto repeal had vanished.

Withholding 10 percent of interest and dividend income had been scheduled to begin July 1, 1983, but was twice postponed - to Aug. 1, then to Aug. 5 - when it became increasingly clear that Congress would wipe the statute from the books.

The banking industry fueled a massive lobbying campaign, flooding the nation with appeals to savers to protest to Congress against withholding.

Setting up the computer software to enforce withholding would cost a lot of money, banks argued. Savers were told they would lose part of their income from interest if 10 percent withholding went into effect.

Such loss would be minimal, according to a Treasury study. A $1,000 deposit earning 9 percent interest, the estimate showed, would lose about 50 cents of yield a year through 10 percent withholding.

Most elderly and low-income taxpayers would have been exempted from withholding, simply by sending an application to their banks.

Terming the bank campaign ''shabby,'' Sen. Robert Dole (R) of Kansas, chairman of the Senate Finance Committee, said the public was being ''sold a bill of goods'' by the banking lobby.

Nonetheless, alarmed depositors - who accept without question that their wages and salaries are withheld - bombarded Congress with preprinted cards demanding that interest withholding be repealed.

The nation's largest banks by and large stayed aloof from the repeal campaign. Some top bankers, whose computers could easily handle the extra chore, told White House officials privately that they had little objection to withholding.

Large US banks are preoccupied by a more fundamental problem, namely their huge and endangered loans to developing countries, especially in Latin America.

Bankers also took note of Senator Dole's warning that the tax status of US banks - which on the whole pay lower effective tax rates than nonfinancial corporations - might be examined by Congress if withholding were killed.

Not all of the $13.4 billion of evaded taxes will be lost over the next five years, because the repeal legislation contains a number of enforcement measures designed to make it harder for taxpayers to conceal interest and dividend income.

Among these is the hiring of additional Internal Revenue Service personnel, who will match individual taxpayer returns with interest and dividend information provided by banks and corporations.