Often used in phrases such as to jump on the bandwagon, meaning to join the popular or apparently winning side, bandwagon denotes a popular party, faction or cause that attracts growing support.

The primary meaning of the noun bandwagon is a usually ornate and high wagon for a band of musicians especially in a circus parade; the earliest instance that I have found is from the New Orleans Weekly Delta (New Orleans, Louisiana) of 15^th February 1847:

Speaking of Messrs Stone and McCollum’s Circus, we are reminded that on Sunday last, the steamboat John Hancock brought down some large additions to their equestrian paraphernalia. Ten baggage-wagons, a magnificent band-wagon, capable of holding twenty musicians, and twenty horses, of the finest breed, came down in one lot.

Bandwagons were also used in other parades, on the Fourth of July in particular, and in political processions, as mentioned in the Cincinnati Daily Press (Cincinnati, Ohio) of 27^th June 1860:

We regret to see, so early in the campaign, such a melancholy expression on Mr. Douglas’s countenance, as is given in the portraits carried in processions in this city. […]
We would suggest also that “Hail Columbia” is a very melancholy tune for political processions. A band-wagon, draped with Douglas portraits, like that which attempted to raise spirits yesterday for the ratification, and playing “Hail Columbia,” appears more like going to bury Douglas, than to praise him.

The following from The Chicago Daily Tribune (Chicago, Illinois) of 26^th October 1879, about “the local Republican ticket”, also evoked the use of bandwagons in political campaigning:

The campaign may […] be quiet, respectable, and comfortable, with a light vote, and, peradventure, a defeat in consequence thereof. If the ticket shall enjoy an easy success, the result will influence the Republicans to proceed in a similar fashion in future local elections; if not, then it will be necessary at another time to revert to the old style of pyrotechnics, band-wagons, vilification, and fanfaronade.

The earliest instance of bandwagon as a political image that I have found is from The New York Times (New York, N.Y.) of 11^th May 1880, which reported that during the Cook County Republican Convention, held at Chicago on 10^th May, Emory A. Storrs had described an opponent within the Republican Party as:

“the chronic political revolver, the political renegade, the bolter of 1872 and prior and intermediate years, the political dyspeptic, the Republican hysteric, the man who was with us in the sunshine and under the band-wagon in the storm”.

The same metaphor appeared in the Mansfield Advertiser (Mansfield, Pennsylvania) on 12^th April 1882:

A correspondent of the Chicago Times says the revolt of Senator Mitchell against the Camerons is looked upon in Pennsylvania as a good joke, and that it is the popular belief that the senator will be found under the band-wagon when the Camerons need him.

On 7^th January 1881, The Cincinnati Enquirer (Cincinnati, Ohio) used a different political image:

We sincerely trust that Mr. Charles Foster will not permit himself to become a mere fifth wheel to the political band-wagon.

The earliest instance of the current metaphor that I have found is from the Woodstock Sentinel (Woodstock, Illinois) of 14^th February 1884, which published an article from a Washington correspondent titled Anything to Beat Hamilton; the journalist interviewed Judge Joslyn, Assistant Secretary of the Interior, who was one of the gentlemen accused of inspiring “the recent dispatch to the Times showing it to be the deliberate purpose of Senators Logan and Cullom to throw Gov. Hamilton overboard and to provide for Mr. Oglesby in such a manner that he will be unable to prevent these gentlemen from succeeding themselves in the senate”; this is an extract from the interview:

“Who do you think will be nominated for governor?”
“While I have no personal feeling about it, from all I can hear I should say Oglesby is the coming man by a large majority.”
“What will be the personnel of the state ticket?”
[…]
“The principal candidates for attorney general are Geo. Hunt, of Paris, Ill., and the present incumbent. I understand that Hunt is running under the wing of Oglesby. If so, he’ll beat McCartney, provided the latter allows Hunt to load him into the Hamilton band-wagon.”
“It is very evident, Judge, that you consider that any man who goes into Hamilton’s band-wagon is liable to get left.”

On 2^nd January 1887, The Chicago Tribune (Chicago, Illinois) used the phrase with explicit reference to opportunism:

The candidates for Speaker are vigorously trying to silence the Senatorial question until after organization (Wednesday), and in this patriotic effort to keep their own interests from being lost in those of the Senatorial candidates, they are being aided by the desire of all new-comers to steer clear of expressions on the successor to Gen. Logan until they see what shape that fight will take and can see their way clear for getting into the band-wagon with the winner.