The expression "jump on the bandwagon" most likely entered popular lingo during the middle of the 19th century, as a reference to the colorful wagon used during pre-circus parades through host cities. Band members would ride at the top of these ornate carriages, accompanied by other performers or privileged local citizens. The parade route would soon fill up with curious spectators, with the hope they would later become patrons of the circus itself.
Jumping on the bandwagon soon became shorthand for supporting a popular cause or political candidate because of a herd mentality or superficial attraction. Congressional records kept during the 1890s reveal several uses of the word during various campaign speeches. The speakers themselves warned voters not to jump on their opponent's bandwagon in haste. Few politicians would ever admit to benefiting from one of their own.
The concept of a herd mentality did not escape the marketing and advertising world, either. In fact, one classic appeal to potential consumers is called the bandwagon approach. One thing this creates is "stickiness" — the natural instinct to join a popular group and remain loyal. Advertisers count on this instinct to promote products with mass audience appeal. The concept behind this approach is to suggest that everyone else is buying this product, so why shouldn't you?
Jumping on a bandwagon is not necessarily a bad thing, as long as individuals join of their own free will. Some political bandwagons do become more of a populist or groundswell movement. But opposing candidates counter this mentality by suggesting voters are too easily swayed by the ornamentation and shininess of the bandwagon, not the candidate or political cause driving it.
Some consider jumping on the bandwagon as going beyond mere support of a candidate into active participation in his or her campaign. Such people often become very vocal supporters, much like the band members on the original circus carriages. The political bandwagon is supposed to generate interest in the candidate through sheer force of personality.
Outside of politics and advertising, the idea of jumping on a bandwagon is not always seen as a positive. Some equate it with a superficial desire to be on a winning side, regardless of one's true personal beliefs. Others see it as a last minute conversion by those seeking safety in numbers.