HARTFORD  It was here in the third-floor billiard room where America’s greatest humorist used to hole up when the bustle of his house, and the nursery one flight below, became too great, leaving strict orders not to be disturbed.

Seated at a desk tucked behind the billiards table, he wrote and rewrote in longhand such masterpieces as “Huckleberry Finn,” often beseeching his editors to stop cleaning up his language and restore the earthier vernacular he preferred.

But now the fanciful, three-story confection that was built by Mark Twain in 1874 at the height of his success may be forced to close because it is running out of money.

In a reversal of fortune that seems ripped from the pages of Twain’s classic “The Prince and the Pauper,” the nonprofit organization that has long run the 14,000-square-foot house as a museum told its trustees last month that it was “facing significant challenges” that could force it to shut down.