MR. REAGAN reached two further levels of irrelevancy and evasion last week in dealing with the Beirut bombing. You will recall he had first attributed the insufficient security to the sort of routine slippage associated with a slow kitchen- remodeling job. Public outrage -- some from his closest admirers -- forced a retreat, and he was soon back with an even more feckless alibi: that the bombing resulted from the "near destruction of our intelligence capability in recent years -- before we came here."

Now, the pros regard penetration of terrorist groups as the best safeguard against the bombings, murders and kidnappings that have dogged Americans in Lebanon. It is conceivable that specific steps taken in the past could have derailed the penetration of the ring that attacked the embassy. But there is not a shred of evidence available that this is so. The testimony of every current and past knowledgeable official, including Vice President Bush, himself a former CIA chief, is consistent on that score. Mr. Reagan's CIA director confirms that the general post-Vietnam cuts in the agency's budget and personnel were reversed in the Carter period.

As the speciousness of Mr. Reagan's attempt to fob off the disaster on Jimmy Carter became obvious, the president went to Evasion No. 3. He complained to reporters about "the way you distorted my remarks about the CIA." A spokesman explained that the president had in mind "reports in the media putting the blame entirely on the Carter administration." We checked the text. It permits no other reading than that he was dumping full blame on Jimmy Carter, to the point of assigning him a motive of guilt about spying. Mr. Carter deserved at least the "explanation" -- not an apology, the White House said -- he got from Mr. Reagan last Friday.

There is a note of real apprehension in the administration's responses, as though it feared that the very word "Lebanon," like "Iran" for Jimmy Carter, is hauling people back to a place of presidential failure and ignominy. There is a hint that the bombing could become the first disaster to break through the president's defenses of staff and style and to mark him personally. His proper response is to acknowledge the emerging truth about the Beirut bombings -- and to do all that is necessary to protect Americans still on the scene.