THE PRESIDENTS explanation at his Thursday press conference of his role in the Marston case is baffling. David W. Marston is the U.S. Attorney in Philadelphia whose prospective replacement by Attorney General Griffin Bell is causing such a furor. At first Mr. Carter said he had not interfered in the case and had not discussed it with the Attorney General. But his answers to further questions made it clear that he had done both. He seemed willing, until he was pressed, to leave the matter resting on those first complete, disingenuous answers. A few months ago the President let his role in the settlement of the Richard Helms case rest on answers that were similary misleading.

We do not quarrel with the rationale on which Attorney General Bell says he himself decided early last year to remove Mr. Marston. Mr. Marston was a political appointee whose claim to the job rested on his ties to a republican senator. He had no track record as an experienced law-enforcement officer. Thus, there was no great reason then for Mr. Bell to resist the pressure he was getting from various Pennsylvania congressmen to fire Mr. Marston.

But much time passed, and many crucial events occurred, between the Attorney General's decision and the steps that were finally taken to put it into effect. The Attorney General, as we understand it, refused to nominate the man the Philadelphia congressman wanted. And Mr. Marston turned out to be a stronger contender than anyone anticipated. He took on the police department, which is one of country's worst. He convicted two of city's most powerful Democratic politicians and a suburban Republican leader. And by the fall his office had under way an investigation into the activities of two Democratic congressmen.

Those facts should have been sufficient to cause the Attorney General either to reconsider his decision to fire Mr. Marston or to worry about the timing of his removal. Unless that removal was handled very carefully, it was going to look like an effort to squelch investigations of this type. But the Attorney General plunged ahead after getting a call from the President, who said a Democratic congressman was pushing him hard to remove Mr. Marston and would now accept anyone as a replacement. That congressman, it turns out, is one of the two under investigation.

The Attorney General's explanation for this is that he did not know until this week that Mr. Marston was investigating members of Congress. That is simply flabbergasting. One would think that the Attorney General would be told of any federal investigation of high political figures. After all, he does have a responsibility to protect the President and himself against entangling alliances with people who may be indicted.

Whatever you attribute these various shortcomings to - naivete, bad briefings, dissembling or caving in to congressional pressure - the picture is not pretty. The implication is there that the President and the Attorney General have been a part - wittingly or not - of an attempt to cover up a Democratic scandal in Philadelphia. Probably the only way the President can keep the situation from getting worse is to keep Mr. Marston in office until the present investigation is concluded - and then to consider the question of replacing him.