I’ve noted many times that polls prior to primaries are of little predictive value. Polls in December 2007 showed that Clinton had a huge lead over Obama. In December 2003, Howard Dean was pulling away in the polls. Eventual winner John Kerry was in sixth place with only 4 percent, even trailing Al Sharpton. While Clinton is certainly in a strong position this year, her leads in the national polls do not guarantee victory. Similarly, while Donald Trump seems to have a significant lead in the Republican race, it is premature to assume he will win unless he actually performs well in Iowa and New Hampshire.
has looked at the December polls and has also demonstrated how little predictive value they have. In polls from November 2007, taken two months before voting, Hillary Clinton was leading the Democratic race. Rudy Guiliani was leading the Republican race, with Fred Thompson also ahead of John McCain. In 2011 Herman Cain was leading the Republican race, and had left the race by the time of the Iowa caucuses.
One reason for the poor predictive value of national polls is that whoever wins in Iowa and/or New Hampshire generally gets a huge boost in subsequent states. That doesn’t mean that the polls in Iowa are all that more meaningful as voters there typically don’t make up their minds until the last minute. Hillary Clinton was leading in the polls in Iowa and came in third place in 2008. In late 2007 Mitt Romney was leading the polls in Iowa, and came in second to Mike Huckabee. Herman Cain was leading in Iowa as well as the national polls two months before the Iowa caucus.
Polls also have limited predictive value as the pollsters do not know who will actually turn out to vote. If the Democratic caucus in Iowa is dominated by long time Democratic voters, then the polls are showing the race as very close. If those who haven’t previously voted but are showing enthusiasm for Bernie Sanders turn out to vote, there could be a big victory for Sanders. The much stronger degree of support seen for Sanders in social media is very encouraging, but no guarantee of votes. It might also be helpful for Sanders that the Iowa caucuses are later this year than in the 2008 cycle, when many college students were off on vacation. Even more might turn out for Sanders in this year than had turned out for Obama.
Similarly we will not know whether Donald Trump will easily win the nomination, or if a party regular will challenge him, until we see how the voters act.