Nearly two months have passed since President Reagan announced his dramatic program for a Middle East peace settlement -- not long for a conflict measured in decades, but long enough to convey an apparhension that much of the momentum may already have been squandered.

When he introduced the program, after his frustrating Lebanese summer, Reagan spoke with decisiveness and vigor. Admittedly, he proposed nothing that was not on the agenda, in one form or another, of earlier administrations. The difference was that this time Reagan seemed prepared to put the power of the United States strongly behind a program's realization.

The president's words upset the equilibrium of forces in both Israel and the Arab world. What he proposed was a political and territorial compromise, reasonable enough to be tempting to both camps. Within each, however, are all-or-nothing forces, bitter enemies but diplomatic bedfellows, and Reagan was challenging a status quo that is dear to them both.

The Begin government reacted first, rejecting the Reagan program outright, but it was clearly stunned by Reagan's move. The Arab extremists, more divided, were slower to respond, and while they were collecting their strength, the Arab heads of state met at Fez and, in effect, invited Reagan to continue with his initiative.

Meanwhile, both camps stayed on the alert. The question all wanted answered was not so much whether Reagan had the power to compel a settlement but, having the power, whether he would choose to use it. He had never chosen to use it before. Had he been so influenced by the events of the Lebanese summer that he would decide to use it now?

Among the Israelis, the debate over the Reagan plan quickly became lost in the greater divisions over responsibility for the Beirut massacres. No doubt the Begin government will do its best to make sure the plan is never heard of again.

Activity has been greater on the Arab side, and, since Fez, the crucial episode was the meeting between Jordan's King Hussein and PLO Chairman Yasser Arafat. The two, who were once at war with each other, explored the prospect of reaching a common position for negotiating on the Reagan program.

The Begin government has taken pains to explain why negotiating with either is impossible, Arafat being a killer and Hussein a weakling. But Arafat, if he is no saint, has said often enough that he will make peace with Israel in return for a West Bank-Gaza settlement that he deserves to be tested. As for Hussein, he has openly held this position for years.

Both, however, know that their leadership is in jeopardy from Arab rejectionists, and neither is willing to throw himself on his spear in behalf of a White House speech. Far from being weak, Hussein would gamble on the Reagan peace program, on the condition that it appear credible, and so would Arafat. But neither will take the risk if the president does not follow up his speech with strong action.

Last week, Morocco's King Hassan brought an Arab League delegation to Washington to explore the Reagan initiative. The president might have signaled a change by allowing the king to include in the delegation a PLO representative, as he had planned. But Reagan vetoed the idea, and offered the Arabs only the feeble homily that the way to start talking is to start talking.

It is not nearly as simple as that. The Begin government has clearly indicated its satisfaction with the current status of the West Bank and Gaza. Reagan has to offer an incentive, whether it be carrot or stick, to persuade Begin to come around to a territorial settlement.

But so far the appearance is that nothing has changed in U.S.-Israeli relations. We backed Israel to the hilt in the recent unseemly squabbles in the United Nations. In contrast, Israeli Foreign Minister Yitzhak Shamir, after a series of meetings here, still proclaimed the same tough conditions for Israel's withdrawal from Lebanon, a crucial objective in the president's Middle East policy. The weak American response to Shamir did nothing to enhance international confidence in the prospects for Reagan's peace plan.

Given the American political system, it may be too much to expect the president to act on the eve of an election. But the hard-line forces on both sides have already regrouped, and Reagan has lost the advantage of surprise that he had a month ago.

The diplomatic terrain is already littered with well-meaning American peace plans, with distinguished names like Rogers, Sisco, Kissinger, Carter and Habib attached to them. All have fallen short of settling the Middle East struggle. Reagan's still has promise, but, unless it gets some serious application of American power very promptly, it will collapse with the others onto the scrap heap of history.