How Accurate Have Senate Polls Been, And What Could That Mean For November?

The 2016 election was an off year for state-level polls of the presidential race. But for polls of U.S. Senate races, it was a perfectly ordinary year — better than average, even. And Senate polls in the 2018 election were even more accurate.

In the 2017-18 cycle, U.S. Senate polls conducted in the three weeks leading up to the general election had a weighted average error of 4.3 points. (As always, we define error as the absolute difference between a poll’s margin and the actual election’s margin between the top two candidates. For example, if a poll gave the Democratic candidate a lead of 2 percentage points but the Republican won the election by 4 points, that would be a 6-point error.) In the 2015-16 cycle, it was 4.9 points. And the long-term average, for all U.S. Senate elections since 1998, is 5.2 points.

In the grand scheme of things, that’s pretty accurate — not as true as polls of presidential general elections have historically been, but better than polls of presidential primaries or U.S. House races.

What’s more, Senate polling has been remarkably consistent in its accuracy from year to year. Since the 2001-02 cycle, the weighted average error of U.S. Senate polls conducted in the final 21 days before the election has never been lower than 4.2 points (its score in 2005-06) or higher than 5.4 points (2013-14). But that doesn’t mean there haven’t been any surprises in Senate races. The battle for the U.S. Senate, of course, is waged on a state-by-state basis, and while polling errors may average out to around 5 points across these races, they are higher in some states and lower in others.

You can see this clearly in the table below, which shows the weighted average error of U.S. Senate polls by cycle for states that had at least 10 polls conducted during the last 21 days of the campaign. (This threshold prevents us from showing states where the sample size of polls is too small to draw meaningful conclusions; it also focuses our attention on the states with the most competitive Senate races each cycle, as close races tend to attract polling interest.) The table also shows the weighted average statistical bias of Senate polls in each state, which tells us whether that polling error went in favor of Democrats or Republicans.