HERE'S A POLITICAL riddle for you: A bomb exploded in Jerusalem last week, killing two Israelis and injuring 30 or 40 others. In Beirut, responsibility for the attack was promptly and proudly claimed by a unit of Fatah, the principal guerrilla group within the Palestine Liberation Organization.The PLO, the umbrella organization of the Palestinian national movement, also includes various other military, political and social groups, among them the Palestinian Red Crescent (Red Cross). Yasser Arafat is the head of Fatah and of the PLO as well. What or who should be blamed for the bombing?

To many, if not most, Israelis and to many Americans, it suffices to blame the "PLO" and "Arafat," used interchangeably, and to characterize the PLO and Mr. Arafat as "terrorist." To be sure, this approach rationalizes Israel's formal refusal to concede that there is a Palestinian national movement and a representative organization speaking for it, but it does more than that. It fairly fixes responsibility for repeated acts of terror. For the PLO by its charter and Mr. Arafat by his practice do sanction armed struggle against Israeli military target.

To others, however - and here we include ourselves - it is not enough to dismiss the PLO and its leadership as "terrorist." They are that but they are more than that. The PLO is a political organization as well as a military one, Yasser Arafat a politican as well as a guerrilla leader. It and he must accept responsibility on the military side, but they must be acknowledged to have a political side, too. This is not to endorse all the PLO's political tactics and goals, but it is to recognize that the PLO does speak for people with legitimate political interests that must somehow be taken into account.

The State Department, we note, went a good distance last Friday in trying to sort this out. Earlier in the week, a department official had stated, in an incomplete formulation, that the United States did not regard the PLO as a "terrorist" organization. That apparent whitewashing of an outfit that, among other things, plants bombs and kills people produced dismay in Israel. Partly to smooth the reception the Israelis were then preparing for Vice President Mondale, the department whipped up a more complete presentation.

A spokesman not only condemned the latest bombing but explicitly extended condemnation to 1) Fatah for conducting the attacks, 2) the PLO for being "an organization, elements and members of which carry out acts of terror" and 3) Mr. Arafat as the leader of Fatah and the PLO. The spokesman also noted that within the PLO there are persons and factions "who do not advocate or condone terrorism," and he said the administration was still ready to talk with the PLO if it adheres to a U.N. resolution affirming Israel's right to exist.

That is not a bad place to let the matter rest while the search for a Mideast settlement goes on. Israelis deserve official American understanding for their ordeal of terrorist fire. They need it especially at a moment when Washington is urging them to consider policy changes that many regard as a grave threat to their security.

But if the administration has offered certain assurances to the Israelis and warnings to the Palestinians on terror, it has kept open a political vista, too. The message is: When Palestinian organizations and leaders are ready to tell their people - and then the Israelis - that they accept Israel as a legitimate and permanent state, then a basis for negotiation will exist. We think it is in the Palestinians' interest to come to that position, and it is in Israel's interest to make sure that those Palestinians interested in negotiations have an Israeli parnter at the table. Otherwise, the terror - and only the terror - will go on.