In an astonishingly reckless act, President Reagan has now demolished the Republican senators' plan to cut the budget deficit. Mr. Reagan didn't like the oil tax or the temporary crimp in Social Security benefits -- the two elements that made the senators' proposal a serious one. Political tradition holds that it's the president's job to lead the way toward a budget. But Mr. Reagan wants Congress to do it. He declines to take the initiative. This is leadership?

When Mr. Reagan took office, the deficit was approaching $80 billion a year, which, as he said at the time, was too high. Now, in the fifth year of his presidency, it is running at about $210 billion. Last summer there was talk from the administration about a big push, once the election was over, to get the deficit down to manageable proportions -- meaning less than $100 billion, where it had been in the Carter years. But the election has come and gone, and Mr. Reagan still shows no taste at all for any drastic decrease in spending.

The budget that he proposed last February for next year called for a deficit down (slightly) to $180 billion, and even that figure was based on a forecast of rapid economic growth. Yesterday the White House reduced its growth forecast, and it's not likely to be the last reduction. Lower growth means lower tax revenues and higher deficits. If you correct Mr. Reagan's February estimates to the growth rates that now seem probable, you will see that the February budget implies deficits remaining over $200 billion for the next three years. The spending cuts that he recommended are barely sufficient to offset the trend of rising deficits that has been built into the U.S. budget by Mr. Reagan's grossly excessive tax cut in 1981.

When the tax cut was being passed four years ago, the administration argued vehemently that deficits would drop to zero because the lower tax rates would set off a gigantic surge of savings, investment and economic growth. All of that has turned out to be dead wrong. As Robert Dunn of George Washington University points out on the opposite page, the indicators of savings, investment and growth have all been approximately the same under Mr. Reagan as under Mr. Carter, or somewhat lower. Meanwhile, the budget deficit is creating its counterpart abroad -- a foreign debt that by the end of the year will be larger than Brazil's or Mexico's. When the foreign creditors decide that it's time for Americans to pay -- and that will surely happen, although no one can say when -- you will know it. As the Brazilians and Mexicans can tell you, it will mean a sudden, severe drop in American standards of living. Then the United States will have to go to work to make its debt payments through export industries weakened by ars of overvalued exchange rates.

Spending cuts alone won't suffice to get the budget deficit under control. It's going to take a tax increase. As long as President Reagan continues to oppose all tax increases, flatly and adamantly, he is opposing all significant remedies for an American economy that is now running dangerously out of balance. He no longer offers a strategy of his own for dealing with the deficit. But as he demonstrated yesterday, that does not deter him from blocking the strategies that others courageously put forward to protect the national prosperity.