ISRAEL'S REQUEST for large, new and in effect open-ended grants, loans and guarantees forces the question of American-Israeli relations back to basics. Not all Israelis are ready for it, and perhaps fewer Americans, but an extra and deepening American involvement of the sort Israel now invites cannot prudently be accepted on any other basis.

The new request rests on an assertion that Israel is, after the United States, the only fighting democracy: the single friend sharing American values prepared to apply its considerable power for shared goals in a strategic region. The few billions Israel needs, its leadership suggests, are peanuts next to the many billions the United States routinely invests in security partnerships elsewhere. The portion of its economic burden that Israel cannot carry alone,it argues, arises specifically from acts of war (1973, Lebanon) and peace (with Egypt) in which the United States had a major interest.

All of which is the truth but not the whole truth. Prime Minister Peres arrived in Washington this week speaking of the economic medicine his new government has already forced Israel to swallow. And free-enterprise advocates in the Reagan administration still need to be satisfied that Israel's heavily state-managed economy is a fit vessel for extra American aid.

But the economics are only a part of it. For as much as most Americans want to see Israel fare well, there also needs to be a wider consensus on foreign policy. The Israelis, being proud people and living in a dangerous quarter, do not want their freedom of action circumscribed. But the United States cannot blindly finance an Israeli policy that raises American risks and costs unduly in the Arab world.

Flexibility -- dread word to Jerusalem -- must become more evident in Israel's terms for departing from Lebanon. Mr. Peres brought to Washington some hard-won Cabinet decisions to improve the "quality of life" for West Bank Arabs. Other West Bank decisions, even harder, must be forthcoming.

To seek a common understanding on this level is a task of many days and many hands. But unless the United States and Israel are prepared to undertake it, with eyes open, there is only transient value and much potential trouble in expanding aid.