"I'VE SEEN a new Walter Washington," wrote columnist William Raspberry and, as most readers know, this was not the sudden testament of some born-again journalist. Mr. Raspberry was suggesting that the coming of a campaign seems to have turned a weary officeholder into a much more vigorous and even decisive mayor. We've noticed it, too, and would add, Amen. Mr. Washington's pace has clearly quickened. He is speaking and acting forcefully. The most notable example came last Thursday evening, hours after a grand jury had indicted Joseph P. Yeldell and Dominic F. Antonelli on bribery and conspiracy charges.

The mayor put Mr. Yeldell, who had been a top assistant and confidnat, on leave and appointed a temporary replacement. There's nothing all that inspiring about this move, to be sure; it is just about what any mayor would be inclined to do in such a situation. But in the past, indecision and blind loyalties have been the trademark of the Walter Washington administration. The excuse usually has been the fact that personnel procedures make any firings difficult in city hall; and the lame answer usually has been to reward administrative incompetence with new responsibilities.

This time, however, without prejudging the legal charges against Mr. Yeldell, Mayor Washington moved quickly to put some distance between himself and his aide. According to news reports, the mayor decided almost immediately that his aide should be suspended; and he apparently spent much of a four-hour session on the matter trying to explain the "realities" of the situation to Mr. Yeldell, who finally decided to take annual leave.

These "realities," of course, existed long before the indictment of Mr. Yeldell and quite independently of the circumstances that gave rise to hes indictment. Indeed, there has been ample justification for several years for permanently removing Mr. Yeldell from all serious administrative responsibilities. But at least now, in the pressure of campaign politics, Mr. Washington has done the right thing. On other fronts, too, he has been showing decisiveness and even some resourcefulness. He's announcing new programs, pushing tax-relief measures and reciting every accomplishmenbt he can think of.

And critics are quick to note that Mayor Washington is only doing these things to get re-elected.

And why not? That's what the elective process is all about. if it takes the coming of an election to jolt an officeholder into constructive acts for at least a while, that doesn't make the acts any less constructive. Less than four years ago, people here were bemoaning the absence of any apparent incentive for the mayor to take charge. It could be, of course, that he is now coming forth with too little too late, that years of inertia in city hall cannot be repaired in a few months. That will be a question for the voters to decide at the polls - and one that will no doubt be the subject of increasing discussion as the campaigns intensify.