"climb on the bandwagon"

 "Climb on the bandwagon" - to join a growing movement in support of someone or something, often in an opportunist way, when that movement is seen to have become successful.

The word bandwagon was coined in the USA in the mid 19th century, simply as the name for the wagon that carried a circus band. The bandwagon was a horse-drawn wagon large enough to hold a band of musicians. 

Circus workers were skilled at attracting the public with the razzmatazz of a parade through town, complete with highly decorated bandwagons.

 In the late 19th century, politicians picked up on this form of attracting a crowd and began using bandwagons when campaigning for office. Political candidates used to ride the bandwagon through town, and those wishing to publicly show their support would climb on the bandwagon.

Although the practice is of some age, the saying itself is first recorded about the Presidential campaign of William Jennings Bryan early in the 20th century. Climbing (or jumping) on the bandwagon was akin to providing your support for this popular candidate

I guess you could say the modern equivalent is the sleek recreational vehicles present-day politicians use to travel about the country electioneering.