Despite our admittedly flyblown canons of dignity and good sense, the jig is not yet up for Sen. Harrison Arlington Williams Jr. In October 1980, he was indicted on bribery and conspiracy charges. Last May, he was convicted. Now Judge George Pratt has sentenced him to three years in the hoosegow (he could have gotten 15) and imposed a $50,000 fine. Still, he remains the senior senator from the great state of New Jersey, and why should he not? He protests that he is innocent. He insists that he was set up; that most of his colleagues would act no differently if lured into such sinful circumstances. I, for one, believe him.

Still, he did commit felonies. What keeps his colleagues from expelling him? The Senate Ethics Committee last Aug. 24 voted to expel him. Somehow the charnel proceedings have been delayed five times since then. Now his case is to go to the floor tomorrow. Some say the senator should resign on his own. They ignore the momentous fact that he protests that he is innocent. In this era of juridical glory, why should an innocent man resign? Doing so would jeopardize his case and leave him lonely.

After all, quite possibly he is innocent. True, anyone who wished to watch the evening news last spring could have seen him on film suavely convincing a simulated sheik of his power to bend laws in his bare hands, but in this great day being caught flagrante delicto is no proof of anything. We Americans have, of late, seen a lot of suspicious behavior on the tube. Just last March we saw a young man ambush the president and three other men. Yet the alleged gunman, John Hinckley Jr., insists that he, too, is innocent. Presumably, were he a U.S. senator he would still have his seat. He has not even been convicted and sentenced. In fact, he has not even been tried!

In earlier times, of course, Williams would have resigned like a gentleman. Simple respect for the authority of the Senate would have induced a man convicted and sentenced for felonious acts to depart. Today, however, there remains not much respect for authority, nor much authority.

Our institutions have been successfully drained of legitimacy by sophists and of efficacy by those judges who dream of perfect justice even as they free the most ferocious thugs. Why, really, should Williams resign? He knows what every petty scamp and every arrant scoundrel knows, namely: if a convicted felon insists on his innocence stubbornly enough, a growing number of softheads will come to believe and even admire him. In time, enough new leaks might spring in the nation's system of justice to overturn the conviction, leaving him scot-free and surrounded by those literary agents who merchant fat-removal manuals and other such best-selling works of art. Then there might be appearances on talk shows and a profitable place on the campus lecture circuit. This steady accumulation of leaks is called judicial reform.

Why, indeed, would anyone in America today admit guilt for anything? The more overwhelming the evidence against the accused, the more plausible his pleas of innocence will be to the multitudes of softheads who inhabit the elite provinces of our society. All that the accused must do is lie long and hard. "We hesitate to impute guilt to anyone," says Prof. Walter Berns, author of "For Capital Punishment," "because we are so uncertain of our right to do so."

What keeps Williams' colleagues from expelling him? His presence obviously brings further disrepute to the Senate. It further delegitimizes an important American institution. The explanation for the senators' hesitancy is provided by a wise Norwegian judge who viewed the present state of criminal procedure in the West and observed, "Our grandfathers punished, and they did so with a clear conscience (and) we punish too, but we do it with a bad conscience."