Jim Caron wrote:

In January of 1873 Clemens wrote two letters to the NY Tribune about
Hawaii and the possibility of the U.S. annexing it, topics being
discussed because Congress was debating a renewal of the reciprocity
trade treaty with Hawaii that favored its sugar industry.  I do not have
the texts of those letters with me, so I can't be sure that the attack
on Harris in the reprint is not from one of the Tribune letters.

The passage in question does in fact come from Mark Twain's letter to
the Tribune, written on 6 January and
published on 9 January 1873. See page 570 of Mark Twain's Letters, Volume 5.

RHH