The first two weeks of April have been poor ones for President Ronald Reagan's foreign policies. The climactic event came on April 10 when 42 members of his own Republican Party in the Senate, led by the party leader Howard Baker of Tennessee, joined an equal number of opposition Democrats in disapproving the mining of Nicaragua's harbors.
The vote may or may not stop the mining, which has also been protested openly and publicly by France and Britain. The vote was an expression of opinion, not an act of legislation. But the vote count was 84 to 12. If the action of some previous president has been so overwhelmingly disavowed in the Senate, no one at this writing has yet dug the event out of the files.
The harbor mining is part of an undeclared and theoretically covert war which Mr. Reagan has been waging against Nicaragua for nearly three years. That war has not toppled the Marxist regime in Nicaragua, or prevented Nicaragua from sending supplies and weapons to the rebels in El Salvador, or persuaded the Congress of the soundness of the policy.
That undeclared war has instead strained alliance relations, raised more doubts overseas about the competence of Reagan foreign policy, and given the Soviets propaganda ammunition which, of course, they are delighted to use.
The Soviets got an extra propaganda bonus when the Nicaraguans took their case to the World Court at The Hague, and the President promptly announced he would not recognize the jurisdiction of that court.
Before the harbor mining story broke at the beginning of this week, President Reagan was already involved in an angry dispute with the Speaker of the House, Tip O'Neill, over responsibility for the collapse of the Reagan intervention in Lebanon.
The President has claimed that lack of congressional support had spoiled the Lebanon affair which had ended in the withdrawal of the US from Lebanon after the loss of 263 Marines. Speaker O'Neill called the President's claim ''despicable,'' saying ''the deaths lie on him and the defeat in Lebanon lies on him and him alone.''
Between the row with Speaker O'Neill over failure in Lebanon and the Senate vote over the harbor mining, Mr. Reagan received from Moscow's new leader, Konstantin Chernenko, another cold turndown on nuclear weaponry. Mr. Reagan had sent private letters to Mr. Chernenko proposing a resumption of talks. The answer was ''nyet.'' It was such a firm ''nyet'' that it presumably ended White House hope for a resumed nuclear weapons dialogue with the Soviets before this year's American elections.
Thus the President is well into his reelection campaign in a condition of political vulnerability in foreign policy. He has had his easy win in Grenada, but not since then has any foreign policy project turned out well.
He had sent a mighty armada to Lebanon. He had put US marines ashore. He trained the Lebanese Army. He was going to create a new, unified Lebanon under the leadership of Amin Gemayel. And he arranged a peace between that new Lebanon and Israel. And then it all fell apart.
None of the above proves that Mr. Reagan's reelection is endangered by foreign policy failures. American elections seldom turn on foreign policy. The condition of the economy just before election day is likely to be the decisive factor. The economy is still improving. The Democrats see no way of scoring heavily off him over domestic matters as long as the economy continues to rise, as it seems to be doing.
But should the economy turn against him, he would be hard put to bring his foreign policy record to his rescue.