Traugott says that generally speaking, voters who identify with a specific party are unlikely to change their vote as a result of polling, but that they might not show up to the polls at all if they see their party’s nominee is disfavored to win. 

“It's often the case in the general election that the polls have an influence in terms of turnout,” he says. 

Michael Dimock, the president of the Pew Research Center, says that in tight races, of which there are many in the upcoming midterms, “it's very easy for polls to ‘pick the wrong winner.’”

“I do think that people should be very cautious in over-interpreting pre-election polls' ability to project an election outcome, whether it's at a national level, state level, a district level, [or] local level,” he tells TIME. 

One of the main reasons for this, Dimock contends, is that even if participants in a survey say who they would vote for, they still might not show up to cast their ballot. He refers to this immeasurable but important factor as “enthusiasm differences.”