Reporters are suspect enough in the eyes of the general public without the recent endorsement of Walter Mondale for president by the Newspaper Guild. Opinion polls for several years have shown the credibility of all the mass media to be quite low. And now comes an act by guild leadership that will cause many readers who are aware of it to ask: ''How can I be sure that any stories I read relating to the forthcoming selection of our next president aren't slanted in favor of Mondale?''
The fact is that guild leadership, without any referendum among the union's 31,000 members, provided the endorsement for Mondale. The union also did this back in 1972, when it went on the record as favoring George McGovern.
Now as then many guild members are protesting this union action. Some don't feel the union should make any endorsement, that it does put guild members who write on politics often in an apparent conflict-of-interest situation. Other guild members would have preferred someone other than Mondale.
The fact is, too, that many working journalists, including many who cover the political campaign, are not members of the guild.
But the voters often make judgments based on appearances. The guild action gives the appearance - to many people, how many no one knows - that the press at large is involved and possibly corrupted by this open declaration of allegiance. And for many people the commitment will only serve as a reinforcement of their view that the press cannot be trusted.
US Rep. Paul Simon (a Democrat and former Illinois state legislator and lieutenant governor) also has long been looked upon as an expert on the subject of ethics in state government.
Recently, in discussing the guild endorsement with reporters over breakfast, Mr. Simon said he really didn't believe that it would cause any slanting of stories. Thus, he saw no problem of ''substance.''
But he said there was indeed a problem of ''appearance'' of wrongdoing that does bear on reportorial credibility.
Also recently, in talks to journalism faculty and students at the University of Illinois, I found the guild endorsement a prime topic. Their concern was whether this might result in some reporters, members of the guild, in particular now tilting their stories toward Mondale.
My answer was that the reporters I knew and traveled with along the campaign trails were all honest professionals who would continue to call the shots as they saw them.
No, the danger lies in what the guild has done to the credibility of the press, already at such a low ebb.
Now, it might be asked: Publishers, through their editorial pages, often endorse candidates; therefore, why not an endorsement by reporters?
It is indeed a burden for a political reporter to bear when his or her newspaper endorses, say, a presidential candidate and that candidate uses that endorsement - in advertisements and on placards, etc. - to help win the election.
An increasing number of publications are staying away from endorsements.
That's all to the good. And the guild clearly should quit giving the impression that reporters have chosen up sides in a race in which the public expects them to be neutral.