THE BEST that could have happened to Mr. Reagan's Mideast plan is that the people most concerned would look at it hard, and that is happening. The new proposals are being sympathetically if cautiously inspected in wide parts of the Arab world, though not, of course, by the self-styled Rejectionists. If things go well, the forthcoming Arab summit will consider the Reagan plan -- and split on it. It is on Israel, however, that special attention is focused, not simply because Israel is a democracy where the policy process is conducted in the open but because Israel is where the plan will stand or fall. If Israel, after studying and debating it, finally says no, the plan is dead. The United States cannot shove the plan down Israel's throat if the Israelis conclude they will have none of it.

This is the light in which to weigh the Israeli reaction. The ruling coalition led by Menachem Begin quickly and predictably set its teeth against the Reagan initiative. But the largest party, the Labor opposition, which governed from 1948 to 1977 and now holds 50 of the 120 Knesset seats, pronounced the plan "a basis for serious dialogue" and called for an immediate parliamentary debate on it.

Such a debate is the only conceivable way Israel can equip itself to deal with change, and it is a major achievement of the Reagan address to provoke it. It is no surprise to find Labor leader Shimon Peres seizing the issue. The American proposals offer a responsible alternative to the Begin policies, and they are grist for the mill of a responsible opposition party, especially one like Labor that believes strongly the Likud coalition has weakened one of Israel's basic security supports, the American connection. Moreover, the American proposals are consistent with Labor's longstanding disposition to trade off territory in the West Bank for peace, to consider partitioning the West Bank with an Arab negotiating partner, and to regard Jordan in the first instance as that partner. A great strength of the Reagan approach is that it not only serves, we believe, Israel's vital security and political interests. It also plugs into a point of view that is already held by a major Israeli party and and that many other Israeli citizens could conceivably come to as well.

In the coming Knesset debate, much will no doubt be said about the fidelity of Mr. Reagan's proposals to the Camp David text. This goes to the crucial larger question of whether and how Israel can count on the United States as times and circumstances change. A range of scenarios will be examined, including the terrible vision projected in the Begin Cabinet's first rebuff to Mr. Reagan: "Were the American plan to be implemented, there would be nothing to prevent King Hussein from inviting his newfound friend, Yasser Arafat, to come to Nablus (in the West Bank) and hand the rule over to him. Thus would come into being a Palestinian state which would conclude a pact with Soviet Russia and arm itself . . . (and) after a while, launch an onslaught against Israel to destroy her."

From a distance, it is very easy to joust with Mr. Begin and to dispose intellectually of such unlikely dragons. On the ground, Mr. Peres and his colleagues have a far more difficult task. Almost every Israeli shares, to one degree or another, the doubts dramatized by Mr. Begin. To avoid being paralyzed by them, people like Mr. Peres must exhibit great political skill and courage.

They must also have outside support. That means a sympathetic attitude to Israel's travail on the part of Americans: much of this is foreshadowed by Mr. Reagan's emphasis on persuading Israel rather than imposing an arbitrary American-made solution on it, on the centrality of Israeli security among American concerns, on unequivocal acceptance of Israel by all Arabs and on direct negotiations as the sole method of diplomatic progress. It also means a forthcoming response to the Reagan plan by the Arabs, including Palestinians, with whom Mr. Reagan is asking israelis to share a common fate.