I suspect this has about as much empirical backing as global warming.
"Bandwagon" behavior was well-known even before modern psychology began to measure such phenomena. The ancient Romans elected their annual officials by reporting to the polling place in the Campus Martius and gathering in roped-off areas reserved for each ancient tribe (there were three): the roping-off (praecinctus) gave us our word for "precincts", although they were more like political wards.
Immediately before and during balloting, people would visit across the ropes and talk about the candidates and watch their progress as the ballots were tallied through the tribes and smaller subgroups (I should imagine they were voting by clans and families, though I don't know that). Thus it became political strategy to try to obtain support in the first groups to be polled, in order to sway those voting later. And yes, bribery was not uncommon in the later Republic. That's the essential bandwagon.
I fully understand bandwagoning. It's manifested itself even in places without the vote, where the warlord perceived to be losing has his allies or underlings turn against him in midst of battle. Richard III was the hapless victim of bandwagonning. The difference between factual instances and the foundationless version being promoted by the media is that the historical examples involved real trade-offs involving life and death.
In fact, a major reason Muhammad’s conquests were so rapid had a lot to do with bandwagonning. Anyone who resisted was slaughtered along with his entire extended family. Ditto with entire cities. That was reason enough to join his war bands, as long as he was winning. Weirdly enough, if we had implemented Muhammad’s policies in Afghanistan and Iraq, they would already be pacified, although I’d expect to hear of Interpol warrants for Bush, Rumsfeld, Cheney and Rice for war crimes.