JERUSALEM — Ariel Sharon's national unity government, which was sworn in Wednesday night, will rely on two key factors for stability and longevity.

One is a close and harmonious relationship between Likud Prime Minister Ariel Sharon and Foreign Minister Shimon Peres, the senior Labor Party minister in the Cabinet.

The other is the continued absence of realistic prospects for negotiations with the Palestinians toward a final peace agreement.

Less tangibly, the unity government will need a great deal of luck, an unpredictable commodity that outgoing Prime Minister Ehud Barak so lacked during his stormy 21 months in office.

To say that Sharon, 73, and Peres, 77, have a long history together is an understatement. Both have been there since Creation — that is, the creation of the state of Israel 53 years ago.

The young Peres was an aide to founding father and first Prime Minister David Ben-Gurion, quickly rising in the 1950s to become director-general of the Defense Ministry. There, Peres — the standard-bearer of today's peace camp — was intimately involved in developing Israel's nuclear potential.

Sharon, a dashing infantry officer, served in Israel's 1948 War of Independence. In the 1950s, he gained fame and notoriety as the founder of Unit 101, an elite commando crew that carried out aggressive and controversial anti-terror reprisal raids across the Jordanian border during the precarious first decade of the state's existence.

In those days, Menachem Begin's Herut Party — which later became the core of the center-right Likud bloc — was a powerless opposition. Politics were dominated by Ben-Gurion's Mapai Party, forerunner of today's Labor.

Rising stars like Peres and Sharon naturally saw themselves as protégés of the "Old Man," as Ben-Gurion was called.

Sharon's unity government, with almost 30 ministers and another half-dozen deputy ministers, will be unwieldy at best, unworkable at worst.

The Labor component, moreover, is beset by internal conflict. Several of the defeated and dispirited party's leaders — Yossi Beilin, Avraham Burg and Shlomo Ben-Ami — oppose unity under Sharon and have opted to stay out of the Cabinet. As the party's leadership battle unfolds in coming months, the ideological and personal fissures between pro- and anti-unity groups likely will widen.

For both of those reasons, the perception of Peres' role will be critical. Any sense in Labor that Peres is being sidestepped or marginalized will exacerbate internal party tensions and strengthen the hands of those calling for Labor to leave the alliance. The government's survival will best be served if Sharon conveys that he and Peres together will become an informal inner Cabinet where key decisions are thrashed out.

That was the recipe for the success of the unity governments that ruled Israel from 1984 to 1990 under prime ministers Peres and Yitzhak Shamir.

In those Cabinets, an inner "prime ministers' club" made up of Peres, Shamir and Yitzhak Rabin took the main decisions, far from the debating-club atmosphere of the full Cabinet.

That inner sanctum never leaked. Though political rivals, its members set aside rivalries to condct the nation's business and preserve their awkward coalition.

Ultimately, the longevity of the 1980s coalitions rested on an absence of progress in the peace process. A near-fatal crisis erupted when Peres, behind Shamir's back, tried in 1987 to negotiate an agreement with King Hussein of Jordan.

The same logic will probably pertain in Israel's present political constellation. Under Barak, Labor lost the election chiefly because of the collapse of peace negotiations with Palestinian Authority President Yasser Arafat and the eruption of Palestinian violence.

The alliance-of-convenience with Likud is predicated, in effect, on the impossibility of reviving those talks. Laborites roundly blame Arafat for their collapse; many have come to believe that as long as he is power there will be no further thrust toward peace. In practical terms, that means Israel's two major parties can work side by side to reduce the current level of violence and to aspire to limited or interim accords with the Palestinian Authority.