There’s a vibrant market in betting on the House and Senate election outcomes. We know Chief Justice Rehnquist organized a betting pool at the Supreme Court on the outcome of the presidential elections.
Does such betting violate federal law? My quick search found a brief reference in the Supreme Court’s Letter Carriers case. It references a 1939 form for federal civil service employees, which provides: “24. Betting or wagering on elections: Betting or wagering upon the results of primary and general elections is penalized by the laws of most States and is improper political activity.” See also this news report indicating such betting is at least nominally illegal in Arizona.