Pole lend themselves to news coverage in which elections are reported as if the most important aspect is which candidate is in the lead.
Horse-Race Journalism
Shift of support to a candidate/position holding the lead in public opinion polls and therefore believed to be endorsed by many people.
Bandwagon Effect
Tendency for respondents and declared voters to tell pollsters what they think the pollsters want to hear.
Social Desirability Bias
Anti-establishment, anti-media attitude that drew these voters to the outsider candidates also turned them away from pollsters.
Non-response Bias