Russia's leaders are a favorite punching bag of the American press. No doubt they have earned many a verbal slugging. Their addition to cynical manipulation of the law and unrepentant contempt for human rights of political dissidents have made them an easy mark for well-meaning Western critics.

Easy sparring partners, however, make bad boxers, and the Kremlin's easy targets make for bad American journalism. Just because, when the Soviets prosecute a political dissident, the prosecutors themselves seem guilty, we've come to assume that the dissident must be innocent. Consequently, the reporting tends to be lazy. It prejudges the Russians, while rightfully accusing them of prejudging their political dissidents.

Was Anatoly Scharansky, who was tried in Moscow for treason, innocent? Our own leaders seem to have proclaimed him so, President Carter called his trial a violation of the Helsinki human-rights accord, and the House of Representatives overwhelmingly passed a resolution blasting his trial as " deplorable." Such official chest thumping has been reported without question in our press.

Since the conviction and harsh sentencing of Scharansky, there has been an outpouring of American editorial outrage. But did our press examine the evidence against Scharansky (to the extent it filtered through Soviet censorship) with a reasonable degree of objectivity?

Unlike Alexander Ginzburg, the Soviet dissident who was convicted of a non-crime ("anti-Soviet agitation and propaganda"), Scharansky was charged with a universally recognized crime: treason. He was accused of passing information about the Soviet defense industry and defense installations to a Western " military intelligence services "through a" foreign correspondent." Is that implausible?

We do know that Scharansky, a computer expert, passed information to Robert Toth, the former Los Angeles Times correspondent in Moscow. Toth has denied that he, Toth, passed along information to western intelligence officials. In a highly argumentative article, he recently wrote that none of the information he received from Scharansky was "secret," but at least one of his earlier published columns was headlined "Russ Indirectly Reveals 'State Secrets.'"

We also know that some American reporters and editors have long served as errand boys for intelligence agencies. (No longer, one hopes.)

We also know that Scharansky had an obvious motive for doing what the Russians say he did. One Sanya Lipavsky, a former Jewish activist and Soviet informer, reportedly testified at the trial that Scharansky's primary goal was "to change the existing system in the Soviet Union." Is that goal implausible? Could it not supply a motive for treason?

I do not suggest that Scharansky committed treason. From this distance the evidence is too fragmentary. But I am far from certain that the Russians knowingly prosecuted an innocent man.

The Soviets can in any event be condemned for secret, unfair trial procedures. But Washington's cries of indignation go much further. They - we - seem to assume Scharansky's innocence.

An American case with disturbing parallels to Scharansky's has just concluded. Ronald L. Humphrey, a middle-level American Foreign Service officer, and David Truong, a Vietnamese graduate student, were recently convicted in federal court in Virginia of espionage for passing so-called documents to the Vietnamese. The documents were classified at only the lowest level - "confidential" - and the defendants claimed they contained no national security information. Morton Halperin, formerly on the staff of the National Security Council, so testified; at least one State Department assessment arguably agreed. However, an articulate general testified that the documents could be viewed as relating to national security, and the "spies" were convicted.

Viewed from Moscow, might not the Humphrey trial look something like Scharansky? Should not we ponder Scharansky's trial in the light of our own government's prosecution of those who pass on debatably "secret" information to foreign powers? Still more important, should we not ponder our prosecution of Humphrey in light of the Soviet prosecution of Scharansky?