WHEN IS A SMEAR CAMPAIGN not a smear campaign?

That's an important question for the voters of 1986 to answer as the midterm elections spend their final week moving gutter-wise.In Maryland, senatorial candidate Linda Chavez spends her days and her exposure raising suspicions about the sexual preferences of her opponent, Rep. Barbara Mikulski, D-Md. Rather than hearing about Ms. Chavez' philosophical differences with her opponent, Maryland votersare learning that Ms. Mikulski is an "anti-male . . . San Francisco Democrat" who once hired a ''radical feminist" and should "come out of the closet."

That's a code it doesn't take the CIA to break.

In Wisconsin, Ed Garvey, Democratic challenger, brings out Ralph Nader, consumer advocate, to attack Sen. Robert W. Kasten Jr., R-Wis. Sen. Kasten ''is a chronic drunk," who should not be making important decisions, Mr. Nader bluntly told a news conference.

In Pennsylvania, meanwhile, the gubernatorial campaign is at the point where the only subject of debate is the campaign itself. William W. Scranton, lieutenant governor and a Republican, promised no more negative campaigning and offered his hand to his opponent, Robert P. Casey. But Mr. Casey, making the high-minded claim that "he started it," declined the hand shake.

Mr. Scranton then sent out 600,000 brochures accusing Mr. Casey of being ''blind to fraud, abuse and corruption."

The latest is that Mr. Scranton apologized for the brochures, calling it a mistake. And Mr. Casey declined to accept the apology and is making new commercials about the brochures. "Campaigns are beginning to be almost 100 percent mud. Dirty . . . campaigns are getting worse and worse and worse, and they're not talking about the issues," summed up Sen. Barry Goldwater, R- Ariz.

But the line is harder to draw than that. If all the voters ever received was each candidate's lofty explications of his or her ideals, they wouldn't know much of anything about the people they had to vote for.

To the extent that the negative campaigning brings to light an important shortcoming of a candidate, it can be useful. Never appealing, but not immoral either.

Using this guideline, it becomes a bit easier to distinguish among the examples set above. Ms. Chavez, having gone the furthest on the least, clearly is in the wrong. It's a campaign of unproven innuendo, and even if there was substance to it, her charges have little to do with Ms. Mikulski's abilities.

The Garvey/Nader charges are tougher. Yes, Sen. Kasten did have a drunk driving arrest last year, at a time when the Senate was debating a bill very important to his constituency. Sen. Kasten has previous driving arrests and has been observed drunk in the Senate, according to Mr. Nader. Mr. Nader's charges may not be nice, but they're certainly relevant, particularly in this

drug-obsessed election year.

Finally, there's the matter of Scranton and Casey. Neither one seems to be running a particularly grown-up or relevant campaign. But to the extent that Mr. Scranton can and has pointed to evidence that backs up his claims of abuse, fraud and corruption during Mr. Casey's tenure as Pennsylvania's auditor general, then the voters should hear it.

Enough of this. Suffice to say that negative advertising, while low-minded and unpleasant, sometimes brings important facts to the fore.

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