The smart talk holds that in the last few years Israel has been relentlessly dissipating its traditional strong support in this country. An accounting would finally be rendered, many people figured (including me), not only by the executive branch, whose broad foreign policy responsibilities almost ensure collision with an Israeli government intent on getting its regional way. There would also be a reaction in Congress, the place in the American government where popular regard for Israel and the influence of Israel's Jewish American supporters have always registered strongly.

So what happened in the first post- Lebanon test of this theory, when the foreign aid bill went before the lame-duck Congress in December? Israel did not simply hold its own and get the $2.5 billion the administration had earlier asked for it. In what amounts to a tremendous increase for Israel at the expense of other would-be beneficiaries, a fifth of that total was shifted from loans to grants.

The president and secretary of state opposed the shift, saying it would send all the wrong signals in the Middle East and make a hash of American policy in other places. The four key committee chairmen agreed--Percy and Zablocki of foreign affairs, Hatfield and Whitten of appropriations. But none carried his committee, and in neither the Senate nor the House was the vote close.

The Reagan administration is reluctant to concede it lost much in December or that the result has any particular implications. But the vote surely marked the most substantial foreign policy defeat the administration has yet suffered. Its plain meaning was to cast a shadow across the possibility that, if he chose to, the president could cut aid in order to move Israeli policy.

Correctly in my view, the administration feels that the moment for raising the aid question has not yet arrived. The possibilities available on the political track, especially in Lebanon, have not yet been exhausted. But if Americans and Israelis alike cannot look down the road and see that the administration has an aid card in its hand, it has to be asked whether the president can ultimately make his peace plan work.

Thinking such thoughts, I paid a call on Sen. Robert W. Kasten Jr. (R-Wis.), the first-term self-styled neoconservative whose appropriations subcommittee gave the extra aid for Israel a crucial boost in December.

The perception that support for Israel had been fading comes from "certain executive branch quarters" and reflects the "frustration" felt in the State Department and the White House, Kasten said. Such "cocktail party conversation is not reflected in the legislature because most of us separate short-term political decisions and policies of the Israeli government from long-term overall support for Israel as our best and most stable ally in the Middle East." What concern people felt about Lebanon and the massacre, he added, was eased by the inquiry.

The Israeli or Jewish lobby, Kasten said, is important in some regions, though not his; its influence is overestimated. Was he concerned about taking a card out of the administration's hand? The legislative branch has its own foreign policy powers. Applying sanctions or punishing Begin, he concluded, "wouldn't work."

Kasten says he supports the Reagan plan. But he sees no inconsistency in taking an approach to aid that makes it easy for Begin to sustain his rejection of the plan. The senator shows no recognition of how Begin uses the flow of aid to remove from his Israeli critics one of their potentially most telling arguments--that his policies jeopardize Israel's vital American connection.

But Kasten's words and the December record do serve as a useful reminder, to those who needed reminding, that even after the Lebanon invasion, Congress will closely scan any presidential attempt to use aid as a lever for peace. Israel's popular favor in this country and the weight of Jewish lobbying, in whatever combination, remain formidable.

As one who finds great sense and hope in the Reagan plan, I deplore a congressional attitude that undercuts it. The administration, though, cannot just mutter to itself and leak word of its frustration. It has got to make the best of congressional hesitation by presenting it to Arabs as a hurdle that they must do their share to help the administration leap--by, first of all, drawing King Hussein to the negotiating table.

Having helped win Israel that sort of opening for peace, Congress would, I think, surely expect the Israelis to take full advantage of it.