At a time when Israel's fragile and battered coalition government of national unity least needs political turbulence, Israeli political weather-watchers are warning of a storm cloud no bigger than the heavy hand of the heavyset former defense minister and architect of the Lebanon war -- Ariel Sharon.

Two events conspire to hasten his political rehabilitation and restore his unique capacity for polarizing and inflaming the Israeli political scene. One is the opportunity for showboating offered by the trial in New York City of his $50 million libel suit against Time, Inc. To nobody's surprise, he has taken to role-playing as defender of Israeli honor in ways that are making him a hot item in the Israeli press and on television and the center of the sort of controversy that the former swashbuckling army general has long been at pains to create -- when it doesn't come naturally.

The second window of opportunity for Sharon could be Israel's deepening economic crisis. The nearly unanimous judgment of Israeli economists and politicians is that austerity measures so far adopted fall well short of what's likely to be needed in the weeks and months ahead. Failure of the economic rescue operation could bring political chaos and economic adversity. That's a scene made to order for Sharon.

He is a walking controversy. Enemies acquired over a turbulent career are as intense in their opposition as his legions of right-wing followers are frenzied in their support. He is given to anti-American outbursts, directed with particular scorn at U.S. policy in Lebanon, at the Reagan plan to revive the Arab-Israeli "peace process," at the inadequacy of U.S.-Israeli "strategic cooperation." He has a driving ambition to be prime minister.

What are the odds? Negligible, you would have thought more than a year and a half ago, when an Israeli tribunal held Sharon "indirectly responsible" for the failure of Israeli forces to stop Christian militiamen from massacring Palestinian refugees at Sabra and Shatila. As punishment, he was reduced from defense minister to a Cabinet member without portfolio.

But Sharon is not easily shamed and, in the eyes of his followers, not easily disgraced. By April of this year, he was in a position to challenge Yitzhak Shamir for the right to be prime minister if the Likud bloc won the parliamentary elections on July 23.

As it was, nobody won big enough to organize a government, hence the power sharing in a government of national unity under Labor's Shimon Peres, and on the understanding that Shamir will take over for the last two years of a four-year term. Those same alternative prospects for Sharon would probably be valid if the present government collapsed under the weight of the economic crisis, forcing new elections.

While almost nobody would write such a script with certainty, Sharon himself would have no hesitancy. He tells visitors he feels "massive support by the people," and he is wasting no effort to build upon it.

Hence the significance of the lawsuit in New York. It is Sharon taking up the cudgels for Israel -- nothing personal about it, mind you, and certainly not for the $50 million he is seeking. He will use any award "to fight libel against Jews and against Israel" and to fight the case against "Arab terrorism during the last 100 years" by underwriting a study of the damage it has done to the Jewish people.

An out-of-court settlement, accompanied by a retraction from Time magazine for the way it wrote about his involvement in the massacres, he would hail as a victory. Even a loss at the hands of an American jury would not keep him from claiming credit for his lonely crusade -- or rob him of his place on the stage and the torrent of publicity at home.

Predictably, for so loved and hated a figure, it won't all be a plus. His loud charges of "blood libel" have provoked the kinds of passions, pro and con, that Sharon has lived by. As Amos Perlmutter, a political science professor at American University, wrote recently in The New York Times: "Historically, ('blood-libel') is associated with Christian attacks on Jews. . . . The blood-libel charge was the most effective and horrible tool of Christian anti-Semitism."

Ariel Sharon has "wrapped himself in the flag of Israel, but he does not wear it well," Perlmutter went on, portraying Sharon as the man who "planned and led the Lebanese war, who misled his own prime minister, misinformed his Cabinet and allied Israel with the Lebanese faction that eventually oversaw the massacres in Sabra and Shatila . . . who ordered the bombing of Beirut, in open disregard of Israel's traditional concern to fight only just wars."

And yet, two years after Sharon sent Israeli forces into Lebanon, election-campaign crowds were hailing him as "the hero of Israel," singing of "Arik, the king of Israel," and shouting their approval of "the man who built settlements in Judea and Samaria (the West Bank) to the glory of Israel."

To those Israelis, Sharon's performance in the New York courtroom is exactly what they want to hear. That's more than enough reason for well-wishers of Israel and of stability in the Middle East to want the present government of national unity to succeed.