KING HUSSEIN is in Washington, still graciously refusing to take the seat at the peace table that President Reagan offered him on Sept. 1. First, says the king, ever the careful bargainer, the United States must get Israel's withdrawal from Lebanon under way and do something about Israel's settlements in the West Bank. Both demands are right and logical. They should have and, we believe, do have the president's support.

But for King Hussein to use them as excuses is another matter. Every new day he stays away from the peace table gives Menachem Begin more time to harden Israel's grip on the West Bank. Nothing could sooner force the internal debate that must precede a change of Israeli policy than for the king to demonstrate that Israelis have a partner for peace on the West Bank. It is for this role alone that American diplomacy has been cultivating him-- without recompense--for years.

It does now seem, however, that Israel's withdrawal from Lebanon is one political step closer. While everyone was urging Israel to drop its demand that the Lebanese come meet in Jerusalem, back-channel talks were already going on between Defense Minister Ariel Sharon and Lebanese reporting to President Amin Gemayel, with American diplomats helping, too. Front-channel talks are now about to begin on withdrawal, border security and political ties. Israel has dropped its Jerusalem demand.

So let's go. Americans, Arabs and Israelis all accept that Israeli withdrawal is linked to Syrian and PLO withdrawal: if Israel leaves by itself, Syria and the remaining PLO men will never get out. But Israel's agreement to withdraw is the political key to withdrawal by the other two. That is a big reason why the United States has borne down so heavily on the Israelis.

If Washington cannot move Israel out of Lebanon, its credibility will be zero when it comes to advancing President Reagan's Sept. 1 proposals. The only legitimate requirement Israel ever had in Lebanon was to see that Lebanese territory not be used for attacks on it. With the PLO gone from southern Lebanon, that danger has disappeared.

The Israelis appear to have gotten the Lebanese to agree to talk about their future ties. This is fine, but only if the Israelis do not use their presence to spin out the occupation, to build more physical facts of the sort that have already led some Israelis to start calling southern Lebanon Israel's "north bank," or to push political demands beyond the capacity of the frail Lebanese government to bear.

Given the splintering of political and military authority in Lebanon, any new Israeli-Lebanese arrangements are likely to be highly unorthodox, at least in the first phase. That makes it all the more necessary to keep attention on the main thing: the israelis should get out of Lebanon--and soon.