A COUPLE of months late, President Reagan

finally wrote the required letter to the Democrats who supported him on the IMF bill last summer. The letter was required because the Republican Congressional Committee had attacked some of them for voting "to loan U.S. taxpayers' money to communist nations." But the letter is a minimal effort, written in a deliberately legalistic style, evasive and obscure where it should have been direct and clear.

The bill--to increase the lending capacity of the International Monetary Fund--is as urgent as ever. But the Republican right wing, responding to the old isolationist impulse, has been carrying on an outrageous campaign against it. Mr. Reagan can't quite bring himself to squelch the fun.

When the House passed the bill, Rep. Phil Gramm managed to attach to it an amendment requiring the American representative to the IMF to vote against any loans to "communist dictatorships." The administration opposed it. So did the House Republican leader, Robert H. Michel, and congressmen of both parties. Shortly afterward the Republican Congressional Committee began mailing its insinuating letters into the districts of some of the Democrats who had stood with Mr. Reagan on the vote. That stopped the bill cold, since not many Democrats are prepared to help Mr. Reagan if they risk being attacked for it by Mr. Reagan's party in next year's election campaign. The only antidote was a letter from Mr. Reagan himself, suitable for reading aloud in campaign debates.

Now, after two months, the letter has arrived, and it only barely meets the standard. It's composed with all the grace of a real estate contract. But it does contain the key statement that limitations like the "communist dictatorships" amendment would unneccesarily tie the IMF's hands, "adversely affecting the interests of the United States."

Unfortunately, there is also another obstacle to the passage of the bill. The chairman of the House Banking Committee, Fernand St Germain, is holding it hostage for a housing bill. By being less forthcoming on the letter than he should have been, Mr. Reagan has increased the price that he will have to pay in terms of the housing bill--if he really wants the expansion of the IMF.

In September, he called the IMF the "linchpin" of the world's financial system. Now, a month later, nothing has changed. The IMF is still the linchpin. And the IMF bill is still bogged down in Congress, where the White House is doing a little to move it forward--but not very much, and not very promptly.