Senate Republicans yesterday abruptly postponed a scheduled vote on the reappointment of Donald J. Devine to head the nation's civil service system after a committee head count showed that Devine apparently would have been defeated.

The GOP controls the Governmental Affairs Committee 7 to 6, but Democrats apparently are solidly opposed to Devine and Sen. David F. Durenberger (R-Minn.) was absent. Moreover, no one appears certain how Sen. Charles McC. Mathias Jr. (R-Md.) will vote. Mathias was not available for comment and a spokesman said it was unclear how he intends to vote. Facing those circumstances, committee Chairman William V. Roth (R-Del.) postponed the vote until after the congressional Easter recess.

In Maryland live more than 100,000 federal employes among whom Devine, director of the Office of Personnel Management, is highly unpopular. But it was unclear whether Mathias would break ranks to oppose a presidential appointment.

Mathias has not announced whether he will seek reelection in 1986, and Devine is among those mentioned as potential challengers. Devine, a former University of Maryland political scientist who became a key Reagan-Bush campaign official in 1980, has run for state comptroller and is popular among conservative Republicans who have long sought to oust Mathias, a moderate.

Sen. Thomas F. Eagleton (D-Mo.) leads the Democratic opposition, contending that Devine has politicized OPM during the last four years, damaged the morale of federal workers, and ignored the wishes of Congress in implementing various employe changes.

Postponement of the vote occurred on the third day of hearings highlighted by sharp exchanges between Devine and Democrats.

Under the Civil Service Reform Act of 1978, which set up OPM, directors serve four-year terms. Therefore, unlike most presidential appointees, Devine must be reconfirmed by the Senate to continue in that job in President Reagan's second term. A majority vote is required for a nomination to be reported to the floor for confirmation. It fails on a tie vote.

A White House spokesman, asked whether Reagan would lobby for Devine's confirmation, said, "It is a known fact the president thinks Don Devine has been doing a good job, and I can't imagine any more than that needs to be said. He has always supported him."

Devine faced hours of intense questioning by Eagleton on his political activities, which included dozens of appearances last fall on behalf of at least 12 Republican congressional candidates in six states.

Eagleton and Sen. Paul S. Sarbanes (D-Md.) accused Devine of hiring political "thought-control officers" and making the OPM a "stable for political hit men" hired more to promote Republican interests than run the civil service system.

Devine, after announcement of the vote postponement, said, "I understand that several members of the committee who are supportive of my nomination were not able to be present . . . . We are confident that favorable consideration will be given to my nomination when Congress returns."

Devine said his behavior was proper under the 1978 act, which he said "clearly and unambiguously" was intended to make the OPM chief "a political adviser to the president." While the Hatch Act bars most civil servants from many partisan political activities, the OPM chief is exempted.

In testimony yesterday, six federal employe union officials called for Devine's ouster, and Rep. Barney Frank (D-Mass.) told the committee that Devine is "able and bright, but by nature a controversialist" who lacks "the degree of judgment and balanced temperament" needed to head the OPM.