Tomorrow's Senate hearing on the confirmation of Donald J. Devine for a second term as chief of the Office of Personnel Management may not be as short -- or as comfortable -- as his aides had hoped.

Although Republicans control the Governmental Affairs Committee, a number of very hostile witnesses are expected. While it is doubtful they can block his confirmation for a second term, they may make it a very blue Monday for Devine, although he seems to thrive on political combat.

Devine headed the merit system agency until last Tuesday, when his four-year term of office expired. He's now a temporary aide (at $72,300 a year) to acting director Loretta Cornelius pending his return to the top job.

Devine's actions at the agency -- overseeing a cutback in nondefense federal employment, trimming health program costs (and benefits) and directing the overall assault on civil service pay and retirement -- has delighted conservatives in Congress, and many taxpayers.

But many federal workers claim that Devine has sent morale to rock bottom. Civil service unions and their congressional allies wish the 47-year-old political scientist would return to the University of Maryland.

Reagan administration officials have high regard for Devine as a political strategist. But many of them wish Devine had taken a crash course in diplomatic skills, particularly when it comes to dealing with Congress.

Devine will probably be reconfirmed. But the White House unwittingly gave his opponents an opening -- and more time to gather ammunition -- when it delayed his renomination until just a few days before his term ran out.