By Karl Kurtz
So I looked it up. Turns out that the derivation of caucus, a term for a grouping of members in a legislature or a segment of a political party, is unknown and may not be Latin at all. Some dictionaries speculate that it derives from an Algonquin Indian word meaning counselor, elder or advisor, or that it comes "from the Caucus Club of Boston in the 1760s, possibly derived from Medieval Latin caucus, drinking vessel."
But even if it were a Latin word, a useful Wikipedia article points out that there are plenty of Latin words ending in -us whose plural in English does not end in -i: campus, bonus, and anus are examples.
Finally, the same article notes that there are facetious -i plural words:
Facetious mock-erudite plurals in -i or even -ii are sometimes found for words ending with a sound (vaguely) similar to -us. Examples are stewardi (supposed plural of stewardess) and Elvii (as a plural for Elvis imitators). The Toyota corporation has determined that their Prius model should have the plural form Prii, even though the Latin word prius has a plural priora, the Lada Priora having prior claim to that name—though the common plural is "Priuses". The Winklevoss twins are sometimes collectively referred to as "the Winklevii".
Maybe my legislator friend can popularize "cauci" as the plural of caucus.