THE URGENCY generated by the Palestinian riots and the Israeli reaction to them has propelled the United States into a new Middle East peace mission. Whether it goes beyond improvising a response to appeals to Do Something is unclear. But a moment when many people are questioning the decency and durability of the status quo is the right time to try. Emissaries have been dispatched to see if something can be gotten under way.

There does not seem to be a ''plan,'' which will set some people to asking whether Washington knows where it's going and others to suspecting that it does. Rather, to get started quickly and, at the same time, to respect the domestic political calendar, the State Department has taken off the shelf some modest ideas representing enough promise of movement, but also enough caution and consensus, to survive the political battering of an election year and to be useful to any next president. The Kremlin's role evidently is to be addressed in the talks the secretary of state is conducting with his Soviet counterpart to plan the next summit.

The new path skirts the PLO, which represents the bulk of the Palestinian people but which the United States keeps at arm's length in deference to Israeli objections to its tactics and goals. American officials seek instead to draw in the elusive but moderate King Hussein and to resume an old game of mirrors on the West Bank -- trying to locate local leaders acceptable to both Israel and the PLO.

The idea is to use the time up to the American and Israeli elections of next November to build the halfway house of Palestinian autonomy contemplated in the Camp David accords, and then to shift to the issue of the occupied territories' final status. Giving autonomy priority is meant to draw in Israel's ruling Likud Party. Offering an early transition to final-status talks is meant to open a door to Israel's Labor Party and, meanwhile, to give Palestinians the assurance they demand that autonomy is not a dead end.

It's an approach of some cleverness, but cleverness in itself will take American diplomacy nowhere. What is needed is a sure and steady hand and sustained high-level commitment of the sort Henry Kissinger and then Jimmy Carter showed to be essential and productive. Mr. Reagan, who gives no sign of being ready to apply his own prestige, and a successor not yet chosen are going to have to be very tough. This means standing up to Soviets and Arabs to ensure that Israel's legitimate security demands are met and taking the domestic heat from Israel's friends, whose agitation will be the sure sign that American policy is getting serious about respecting Palestinian self-determination too.