Which US political scandal involves Harold Levinson in Downton Abbey's fourth series?

The US political scandal involving Harold Levinson in Downton Abbey’s fourth series is the Teapot Dome scandal.

Harold Levinson, Martha’s son and Cora Crawley’s American brother, is described as facing legal trouble over oil investments. The storyline connects him to the real Teapot Dome scandal, a major American political controversy of the early 1920s. In the show, Harold’s difficulties form part of the plot surrounding the Levinsons’ visit to London and Rose’s social debut.

The historical scandal involved the secret leasing of federal petroleum reserves at Teapot Dome, Wyoming, and two other naval oil fields. Secretary of the Interior Albert B. Fall arranged the leases to private oil companies without competitive bidding and accepted bribes. The affair became public during Warren G. Harding’s presidency and led to investigations, convictions, and the first Cabinet-level prison sentence for a US official.

It is easy to confuse this answer with Watergate, the 1970s scandal that forced Richard Nixon’s resignation. Profumo and Marconi were British scandals, making Teapot Dome the only fitting choice.