Sen. Charles McC. Mathias Jr. (R-Md.) -- considered the pivotal vote on the Senate Governmental Affairs Committee -- has prepared a constituent letter suggesting that he will vote not to reconfirm Donald J. Devine as director of the Office of Personnel Management.

An aide to Mathias said yesterday that the letter was drafted last week but was withheld after Devine, whose four-year term expired in March, suddenly resigned from an interim position as special assistant to OPM's Acting Director Loretta Cornelius. The aide said Mathias wants to wait "until he is in possession" of all the facts surrounding the resignation and renomination before he decides what to do. However, at least one or two copies of the letter were inadvertently sent out to persons who had written to Mathias asking where he stood on Devine's reconfirmation. A copy of one of those letters has been obtained by The Washington Post.

Although Republicans have a one-vote majority on the committee -- which must act on Devine's renomination -- they have been delaying a showdown vote because they are unsure of Mathias' position. All six Democratic members are expected to vote against Devine on grounds that he "politicized" the merit system agency by campaigning last year for Republican candidates for the Senate and House.

Devine defended his action saying that as a political appointee he is entitled to make the appearances and that his predecessor did the same on behalf of Democratic candidates.

There has been considerable speculation in Maryland political circles that the conservative Devine, a former professor at the University of Maryland, might run against the liberal Mathias next year if his reappointment falls through. Mathias has not stated publicly whether he will vote against Devine. Mail and phones calls to his office have been heavily against Devine. Insiders indicate that Mathias' personal preference is to oppose Devine, but that he would clearly prefer the White House to withdraw the nomination.

But in the letter -- which mistakenly went out to one or two Maryland residents before it was put on hold -- Mathias strongly indicates he will vote against Devine. It says that Devine's record at OPM "stands in sharp contrast to the record of the development of the civil service. The issue is therefore cast in pragmatic rather than the more usual personal terms. It is simply put: does the Senate condone a partisan leadership of the civil service that tends to diminish its stature, or does the Senate still support the historic concept of a talented, politically neutral, career service.

"I . . . am convinced that a non-partisan career service is essential. It does not seem reasonable that such a service can be maintained for long if it is not inspired by the example of its leaders.

"Judging by the evidence of his conduct of his first term," the letter concludes, "Mr. Devine does not share this conviction. To vote to confirm him again would, therefore, be to condone what I have condemned with respect to the civil service."

Devine said yesterday that "if he Mathias disavows sending the letter out it appears to me they are backing off . . . a lot of letters get written for the boss that the boss doesn't see, or send out." Devine says he still has the president's backing for the OPM job. "I expect to be back in a couple of weeks. When he gets back from Europe the president is going to talk with Sen. Mathias. Sen. Mathias is a very prudent man and I'm sure he is not going to make up his mind until he talks with the president."