Common Cause President David Cohen charged yesterday that "one overriding weakness" of the Carter administration "is that the Justice Department has been heavily politicized."

Cohen, who heads the 250,000-member citizens lobby, told a group of reporters that President Carter has reneged on his campaign promise to create a merit system to select federal judges and prosecutors.

Reflecting White House policy, the department has failed to push for such a system, he said.

Cohen also asserted that Attorney General Griffin B. Bell "was a part of his [Carter's] campaign" and that the department has not "sorted out the distinction between being the President's lawyer and the people's lawyer."

Cohen criticized Deputy Attorney General Peter F. Flaherty for urging Congress last month to exempt lobbyists' dealings with the executive branch from a bill that would toughen reporting requirements for lobbyists.

Justice Department spokesman Robert J. Havel said later in the day, "We are modifying our position. We will not oppose including the executive as well as the legislative branch in the bill."

Havel also said the department has encouraged the development of a merit system for judges and U.S. attorneys.

He said Bell had clearly defined his campaign aid to Carter during his confirmation hearings in January. That included giving a $1,000 donation, writing two speeches, and aiding in the selection of the vice presidential nominee.

In selecting federal judges and prosecutors, senators belonging to the party in power now have almost unlimited power to pick, or at least veto, nominees that the President recommends to Congress. During the campaign, Carter pledged a system of merit selection "without any consideration of political aspects or influence."

However, after discovering little or no willingness on the part of senators to give up their power over judicial appointments, Carter issued an executive order Feb. 15 setting up advisory panels to recommend nominees only for the 11 federal circuit courts and not for the 400 federal district judgeships or the 94 U.S. attorney jobs. For the district judges and prosecutors, Carter asked senators to set up merit selection panels voluntarily.

Cohen said the selection procedure for circuit court of appeals judges "is so week it dosen't satisfy any standard" because the names of prospective appointees are not made public before they are recommended and because the panels are selected only as vacancies occur and not for specified terms."So you can stack the deck" on the selection of nominees, Cohen charged.

Havel said the circuit court advisory panels are bipartisan and have substantial numbers of women and non-lawyers. The Justice Department has provided advice to senators who want to set up advisory panels for the other legal jobs, he said. Senators in 10 states have set up panels to help select district judges, and senators in five of the 10 have set up panels to help select prosecutors.