The story
US 1899. A bandwagon carried the musicians at the head of a parade or at a political rally, beckoning others to follow. When used to refer to politics, jumping on the bandwagon suggests following the crowd for the excitement of the event rather than any firm conviction in its direction or truthfulness. The phrase is first attested in a letter by Theodore Roosevelt in 1899: : When I once became sure of one majority they rumbled over each other to get aboard the band wagon.
Etymology from Wiktionary, CC BY-SA 4.0.