POOR JASON ROBARDS - until Thursday night, in his Richard Monckton impersonation of Richard Nixon he was doing just fine, the only competition being the other networks and a few superstars like George C. Scott and Ann Bancroft. But Thursday night was different. Monckton-Nixon was up against the genuine article: Richard M. Nixon himself. And we're afraid the comparison was not merely invidious; it was fatal. After an hour and a half's display of the real Richard Nixon's mind-blowing arts and wiles and self-deceptions and delusions and non sequiturs, capped by a disquisition on John and Martha Mitchell that would have been the envy of Uriah heep, Mr. Robards' Monckton-Nixon figure looked the picture of intellectual honesty and moral probity to us - at least in comparison with what had gone just before. So the old truth, we fear, remains: Where Richard M. Nixon is concerned, art can't hold a candle to life.

The interview was, mercifully, the last in the Frost-Nixon series. It was also something of a salami, having been made up of assorted sweepings from the abattoir floor after the earlier scheduled interviews seemed to show a lot of promise. That accounts for the thither-and-yon quality of the subject matter. But it does not of course account for the oddity of what was said. For that we have to hold the principals responsible.

Mr. Nixon was at the top of his form, just as you might say Gloria Swanson was at the top of hers in "Sunset Boulevard." He is getting better every day at having it both ways. Of his gymnast-secretary Rose Mary Woods and her famous Wallenda-like attempt to transcribe some of the tapes while talking on the phone (or so she said), Mr Nixon tells us that Miss Woods was 1) too virtuous to have deliberately erased the notorious 18 1/2 missing minutes and 2) too worldly wise to have stopped at that erasure if she had really set her mind to it. Likewise in his evident eagerness to show that he himself, though innocent, was no dummy, Mr. Nixon made some kind of confession of criminal intent. He would have destroyed the tapes, he said, if he had thought they contained evidence of the crimes that ultimately brought down his administration. Similarly, John Mitchell, that saint of a man who is currently doing time in the federal pen, was just too darned smart to have let all those ridiculous Watergate things happen - and wouldn't for a moment have done so if he had not been preoccupied with the mental health of his wife Martha ("God rest her soul"), who was so mentally disturbed that, among other things, she "said that she was going to blow the whistle on everybody."

None of these professions, explanations and analyses, of course, bears even remotely on the evidence that was gathered over a two-year period and which resulted in the jailing of many top bananas in the Nixon administration and the forced resignation of the President himself. But never mind.To us the high point in this last of the series of lucrative exchanges between Mr. Frost and Mr. Nixon came when interlocutor Frost supplied a not-very-funny jest that, naturally, Mr. Nixon missed. But Mr. Nixon so strained to understand it as serious comment that Mr. Frost had to keep trying to cue him right there on the screen. And then, finally, you heard it in a loud stage whisper, Mr. Frost admonishing the former President: It's a joke! And so it was - the whole thing. We'd be hard put ourselves to think of a better epitaph for Mr. Nixon's televised return to public life.