Election polls especially suffer from two specific types of measurement error: (1) election salience among voters at the time of the poll and (2) strategic voting decisions at the time of the vote which are at odds with poll responses.
On point one, the research literature shows that the farther a poll is out from election-day, the more error prone it will be. Many explanations exist, but the most common one relates to diminished election salience among voters at the time of the poll. Put differently, at the early stages of the electoral cycle, people are not paying attention to the candidates and issues. CONT.
Clifford Young, Ipsos Public Affairs