IS JUSTICE DEPARTMENT picking on Rep. Charles C. Diggs (D-Mich.) by prosecuting him for alleged congressional-payroll abuses? Some of Mr. Diggs's supporters think so, and his lawyers suggested as much in a pretrial motion last week. They claimed that some of the conduct Mr. Diggs is charged with "was relatively common" in the House. And they asked Distict Court Judge Joseph C. Waddy to see if Mr. Diggs had been improperly "singled out" for indictment on those grounds.

Because such allegations have been floating around, it's useful to look carefully at what the motion is grounded on. First, it concerns only the charges that Mr. Diggs paid official salaries to people who did no official work - specifically, his family's lawyer and accountant and an employee of the family funeral home. The counts alleging kickbacks from other employees are not involved in this round.

Second, the brief is more artful than the average "everybody does it" argument. Mr. Diggs is not saying that he did it. Nor are his lawyers really charging that most congressmen do. They mayh yet try to pin down how many representatives use staff members to handle personal accounts, run private errands or chauffeur their children around. For now, though, they cite only the fact that former representatives Wayne Hays (D-Ohio) and James Hastings (R-N.Y.) and Rep. John Young (D-Tex.) have been investigated but not indicted for diverting staff to personal services.

By any normal way of counting, three examples hardly make something "common" in the 435-member House. If one wants to argue by insinuation at all, it's easier to say that Mr. Diggs's lawyers are putting their client in a curious camp. Rep. Hays abused his prerogatives so grossly that he finally left Congress in disgrace. Mr. Hastings was convicted for taking kick-backs from his staff, and the Justice Department is now suing him both for the sums kicked back and for the salaries allegedly paid to employees who worked in his insurance business. So that analogy would seem especially unhelpful to Mr. Diggs.

To Mr. Diggs's lawyers, though, what matters is the Messrs. Hays, Hastings and Young were not criminally charged for using staff wrongfully - but Mr. Diggs was. They want to know why. It's an interesting question; the answer that immediately occurs to us is that the evidence against Mr. Diggs could be uncommonly strong. (Perhaps when investigators looked into Mr. Hays's employment of Elizabeth Ray they found she had done some offical work after all.)

That's not the answer the Diggs team has in mind. They run through some fancy rhetoric about the "chilling effect" of selective prosecution of "troublesome" congressmen. And they suggest that Mr. Diggs may have been "singled out" because he "frequently comes into conflict" with the administration.

That strikes us as the silliest part of the whole argument. As chairman of the Africa subcommittee, Mr. Diggs has been best known for frequent trips to Africa. He has used his House Distict Committee chairmanship in large part to maintain a sizable staff. Even his lawyers cite only one specific conflict: U.S. Attorney Earl Sibert's dissent from the report of a D.C. criminal-code commission that Mr. Diggs backs. That hardly supports the claim that Mr. Diggs is high on some sort of Carter administration enemies list.