PRESIDENT REAGAN would accomplish a lot by vetoing the budget-and-debt legislation that Congress has sent him. Most of it would be unintentional. A veto would give interest rates another push up, and the dollar's exchange rate another push down, as investors saw that he was blocking this year's attempt to hold down the budget deficit. A veto would damage the chance of any serious cooperation with Japan and Germany to avoid a world recession, since cooperation requires the United States to keep a grip on its deficit. And a veto would make next year's campaign a little more difficult for those Republicans who are already on the defensive about the deficit, and who voted for this bill.

One part of the bill would raise the legal limit of the national debt, and that's why it has to be resolved quickly. Without legislation, the Treasury will default on the securities coming due next week. Another part of the bill -- attached to the debt limit precisely to force this issue -- installs an automatic budget-cutting machine. Either the president and Congress agree on a mixture of tax increases and spending cuts to lower the deficit by $23 billion in the next fiscal year, or the bill will impose across-the-board reductions on most categories of spending, including defense. It's designed to force Mr. Reagan to accept either higher taxes or lower defense spending, both of which he adamantly opposes.

But that's a necessary choice, and Congress is right to press him. Even under this bill, the amount of deficit-cutting in prospect is very modest. Congress' budget resolution, passed last June, was to have reduced the deficit by $37 billion. That target has now been shrunk to $23 billion -- a figure apparently not large enough to prevent the deficit from beginning to rise again in the next fiscal year, which, incidentally, starts on Thursday.

The deficit has dropped very fast over the past year, from $221 billion in fiscal 1986 to perhaps $155 billion or even less this year. Revenues surged because of the Tax Reform Act; people rushed to cash in capital gains before the tax preference expired. But the surge is ending, and without further legislation, the Congressional Budget Office estimates, next year's deficit will rise again to about $183 billion. Even with Congress' $23 billion reduction, the deficit is likely to be somewhat larger a year from now than it is currently.

Mr. Reagan doesn't like the decisions that this bill requires, but they are not really hard ones. A veto risks serious financial trouble for his administration and for the country. The safe way out is to sign the bill, and then sign the small but useful tax increase that will follow