Focus of Higgins yarn is back-room politics; A Choice of Enemies, by George V. Higgins. New York: Alfred A. Knopf. 377 pp. $ 16.95.

Bernie Morgan, speaker of the House in the Commonwealth of Massachusetts, is telling a visitor the difference between blue-blooded politicians and politicos like him:

''We almost never hear from Alexander Hamilton in here. John Hancock, I guess he must be devoting all his time to the insurance business these days. . . . As long as I've been in this place, and it is quite a while, I haven't heard a single word from Plato about this year's budget deficit for human services. . . . We didn't come here to pose - we came here to make deals.''

Morgan and the other characters in ''A Choice of Enemies,'' George V. Higgins's 12th novel, rarely go this long without uttering a profanity, nor are they usually so witty and polite.

No, the characters are familiar Higgins types, operating corruptly, from the back room.

''A Choice of Enemies'' is vintage Higgins - very good dialogue and lots of it, inside stuff, and some very memorable characters.