Who would have thought Mark Felt and Mark Twain would have had so much in common? True, Hal Holbrook’s other indelible role, that of the shadowy Deep Throat in “All the President’s Men” has been in the news lately, serving as an example of how sometimes reality can let us down. For those who anticipated something a bit splashier, the unmasking of the real political whistleblower has been rather … anticlimactic. But so too is Mr. Holbrook’s recent rendition of “Mark Twain Tonight!” his long-running, signature show, which has attained a nearly mythical status of its own.

Folksy, irritable, tricksy, and wise, the persona Samuel Clemens created as Mark Twain shows no sign of losing his relevance or his edge. Dazed with affection for his trapezoidal mustache and twinkly eyes we may be, but a talent for needling the complacent makes him an uneasy companion. Schools are still getting antsy about “Huckleberry Finn,” and his use of humor while addressing racial issues can bother the life out of some. Impatient with hypocrisy, toadying and the party line, Clemens cut through what he called “petrified thought” to let the bracing air of reason in – not bad for a guy who died years ago.

In Mr. Holbrook’s full-length, two act evening of Twain-iana, we see how accurate his 100-year-old diagnoses can be. Wandering between a podium and a table, dressed in the iconic white three-piece suit and lighting a succession of cigars, he urges us on to independent thought. In a homespun tale about a bluejay – a wandering shaggy bird sort of tale – the whole jay kingdom comes to laugh at one of their own. Mistaking a hole in a roof for a hole in a tree, the bird tries to fill up an entire house with acorns. Pushing good acorns after bad serves as a handy metaphor for government work, and Mr. Holbrook’s Twain invites us all around to have a good crow about it.

No one on earth is better qualified than Mr. Holbrook to take on the crusty Hannibal native. Since he was 29, half a century ago, he has been refining, developing, and performing his embodiment of Samuel Clemens, commanding a daunting body of text. There’s plenty to memorize, since, according to the Mark Twain Project at Berkeley, their archive contains 600 unpublished literary manuscripts alone. In an accurate reflection of this deluge of material, Mr. Holbrook lists in the program more than 70 snippets of Twain’s writings – any selection of which he may choose to perform. Every performance is, therefore, different, with his choices responding to the climate and the news of the day.

Apparently, the failures of our press and our partisanship were bothering Mr. Holbrook the night I saw him – on Tuesday, nearly every anecdote, every slice of invective took either journalists or Congress to task. He dishes it out with an even hand – neither Republicans nor Democrats seem to be in his good graces – which draws applause and hisses from the targeted groups. The things you learn at the theater: It seems that Republicans like to sit in the mezzanine right, whereas Democrats cluster in the orchestra left.

If a man blows his brains out, it is only so he can be a more efficient Congressman. If an Italian guide once showed him two skulls of Christopher Columbus (one from when he was a boy, one from his later years), it is to illustrate the ease with which the politicians lie. In fact, the dogged pursuit of these themes served to slow the evening – his purpose first grew obvious, then repetitive, and then a little wearying. No question about it, Mr. Holbrook can spin a yarn, but when he fails to vary the color of the threads, the whole thing weakens.

After a first act that suffered from a spotty microphone, Mr. Holbrook sounded in fine fettle throughout the second. When he really gets on a roll, as he does in the extended passage he quotes from “Huckleberry Finn,” one can nearly forget that Mr. Holbrook has now passed Twain in age. Age makeup that once took four hours to apply now has less work to do – the young man who once had to shelve the part because it was typecasting him as a fogey has now turned 80. The piece has slowed, though, and what once set the pace for one-man performance has now drifted to the middle of the pack. Go and enjoy it, though. It represents a slice of history – not just that of the riverboat writer, but of Mr. Holbrook’s own landmark achievement.

Until June 26 (256 W. 47th Street, 212-719-4099).