The unmasking of Sen. Joe Biden as a rhetorical kleptomaniac somewhat diminishes his claims as an American Demosthenes. But that is, in some ways, the least of the problems.

Biden has only occasionally credited orators such as Neil Kinnock (leader of the British Labor Party) and the late Robert Kennedy and Hubert Humphrey for winged words he has borrowed from them. But the real problem, surely, is that these words were in many cases the work of ghostwriters in the first place.

The Post quoted Adam Walinsky, a sometime Robert Kennedy speechwriter, who remembered sitting under a tree in 1968 and writing the lines (mocking Lyndon Johnson's boasts about the GNP) that Biden filched.

''It's a counterfeit of emotion,'' Walinsky said. ''The person who just grabs somebody else's thoughts that way isn't going through the emotion to produce them.''

True, but Walinsky misses a capital irony. If he, not Kennedy, wrote the lines, then they weren't Kennedy's either. And it is far from unimaginable that the emotions were counterfeit in Kennedy's case as well.

And that gets us a bit closer to a useful point. The Biden plagiarism affair might serve a cleansing purpose in politics if Biden's habit were seen as the latest manifestation of a deepening rot in our public discourse.

The public figures who still write for themselves seem to be a shrinking minority. The New York Times Book Review recently carried a fascinating account of how Lee Iacocca's best-selling book was proposed, designed and manufactured for him by publishers and a ghostwriter (who went on to write a book signed by former speaker Tip O'Neill).

Similarly, political oratory has become little more than a tawdry process of passing shopworn phrases from mouth to mouth, like a sort of communal toothbrush -- or, to alter the metaphor, like rancid wine in a new goatskin -- every four years.

For Democrats, the prototype is a dim and distorted memory of John F. Kennedy, who for most of his tragically abbreviated political career was an endearingly awful speaker, mouthing choppy phrases in a Boston brogue, accented with choppy gestures. His most memorable utterances (including the 1961 inaugural address and even his Pulitzer Prize-winning book, ''Profiles in Courage'') were in large part written for him by others.

Biden apologists note that even Ronald Reagan has occasionally lifted a line or two without attribution, including Franklin Roosevelt's line about ''a rendezvous with destiny.'' But of course that line (like, say, ''fourscore and seven years ago'' or ''entangling alliances'') is so embedded in our folklore that every politically literate hearer immediately recognizes the allusion -- which cannot be said of Biden's unconfessed borrowings from the speeches of Kinnock, or even Humphrey.

To find a presidential-class orator who wrote most of his own lines, you'd have to go back to Woodrow Wilson and before him to Abraham Lincoln, the only writer of stylistic distinction ever to occupy 1600 Pennsylvania Ave. And before that, the certifiable masters of their own words were few: the two Adamses, Jefferson and Madison.

But the problem now, as manifested in the Biden matter, has sunk well below mere ghostwriting. In the past, you could safely assume that borrowed lines accurately reflected the thought and character of the speaker. No longer. Biden, snitching Kinnock's tear-jerker lines about his long-suffering ancestors in the coal mines, was simply engaging in a flimflam. No Biden ancestor is known to have been a miner, though one was a mining engineer.

In fact, all this hand-me-down stuff is no more a reflection of the speaker's character than a play script is of an actor's. It is calculated to sound the cliche's of the hour, to create an effect, to manipulate emotions. And often the effect is pernicious, inflating public expectations well beyond the capacities of anyone so essentially characterless as to parrot the speeches of others.

Far from being inspiring, it is not far short of political decadence.