The American Bar Association is taking on the Carter administration in a rare public fight over a politically connected choice to fill a federal judgeship in Iowa.

ABA officials are scheduled to testify today, at a Senate Judiciary Committee hearing, against the nomination of Donal E. O'Brien, a Sioux City attorney who was President Carter's campaign manager in Michigan in 1976 and is now a member of the Democratic National Committee.

O'Brien was found "not qualified" in two separate ABA investigations over the past year, bar association officials said yesterday. The prime issue raised against the administration candidate was not his politics, but his conduct in a murder case 23 years ago, they said.

The defendant in the case was sentenced to life imprisonment in 1955. But the conviction was overturned in 1972 because it was determined that he gave an involuntary confession after being drugged in a mental hospital.

O'Brienn was the county prosecutor at the time, and the ABA charges, in testimony prepared for today's hearing, that the nominee gave its investigators "evasive" answers about whether he received a 1955 letter from the institution informing him that the suspect had been drugged when he confessed.

O'Brien's version of the case "was sharply contradicted" by other lawyers involved, according to the testimony, and the case file in the prosecutor's office disappeared.

In addition, the investigations, by Thomas E. Deacy Jr. and Don H. Reuben, found that O'Brien used tactics "that might possibly be appropriate to a highly charged political campaign," but not to dealing with other attorneys.

The nomination also received wide publicity "suggesting overtones and innuendos of political favoritism and politics in connection with the federal judicial appointment process," the ABA said.

Justice Department officials countered yesterday that O'Brien was one of five candidates selected for the job by a judicial commission in Iowa, and that the state bar association also support the nomination.

Justice spokesman Terry Adamson said that both the state bar association and Attorney General Griffin B. Bell considered O'Brien's role in the drugged confession case before approving the nomination.

This is the first instance of the ABA testifying against a Carter administration judicial nominee. "We think this is of national concern," one ABA official said yesterday. "If they can get this one (nomination) down, what's going to happen with those 152 new judgeships?"

Congress is near passage of a bill that would give Carter the power to name 152 new district and court of appeals judges - the largest patronage package in federal judicial history.

The 55-year-old O'Brien was U.S. attorney in Iowa during the Kennedy and Johnson administrations.