Democrats eye midterm gains as past polling errors temper expectations

Democrats enter the US midterm elections on 3 November with polling that suggests they could regain the House of Representatives and compete closely in crucial Senate contests. President Donald Trump's low approval ratings and public frustration over food prices and household bills have strengthened the party's expectations, although past polling failures offer reasons for caution.

Surveys put Democratic candidates ahead in substantially more than the 218 seats required for a House majority. Several Senate races also appear competitive in territory Republicans have previously won comfortably. A recent New York Times poll showed a tie in South Carolina, which Trump carried by 18 percentage points in 2024. CNN surveys placed Democrats ahead in Texas, Kansas and Maine.

The record of recent presidential polling complicates those findings. Trump exceeded predictions when he won in 2016 and 2024, while his narrow loss to Joe Biden in 2020 followed surveys suggesting a clearer Democratic victory. This year's Democratic primaries also produced surprises. Abdul El-Sayed won Michigan's Senate primary by 1% after polls indicated a double-digit lead. In Wisconsin's gubernatorial primary, a candidate whose polling advantage approached 20 points narrowly lost.

Clifford Young, president of Ipsos Public Affairs, says reaching a representative sample of Americans has become harder as people give up landlines, screen calls and decline to participate. He says conservatives, particularly those on the far right, are especially reluctant to engage with institutions. If polling models fail to account for missing Republican respondents, the results can favour Democrats. Unexpected turnout among Trump's supporters, including people who rarely voted, has also helped explain earlier misses.

Even an error of a few percentage points can change the apparent outcome of a close contest, and several important Senate races fall within that range. Midterm general election surveys have nevertheless been more accurate in recent years, partly because Trump was absent from the ballot. In 2022, polls indicated Democrats would avoid heavy losses despite Biden's weak approval ratings. The party narrowly lost the House but retained the Senate. Young says volatile primaries, where voters often decide late, do not necessarily signal problems with general election polling.

History also offers Democrats encouragement because the president's party commonly loses ground in midterms. Republicans gained more than 60 House seats in 2010 and advanced in the Senate amid a difficult economic recovery and conservative opposition to Barack Obama's healthcare changes. Polls anticipated Democratic losses, though they underestimated their scale.

Several factors could limit Democratic gains this time. Much of the contested territory backed Trump easily two years ago. Changes to House district boundaries have reduced the number of seats that change parties, while deeper political polarisation has left fewer swing voters. Republicans have also benefited from a financial advantage, giving them hundreds of millions of dollars for attack advertising during the campaign's closing weeks.

Polling averages can provide a more accurate guide than individual surveys, each of which carries statistical uncertainty. Even a Democratic polling advantage overstated by amounts seen in recent presidential elections would still suggest a favourable night for the party. Errors can also run in the opposite direction: Democrats exceeded most surveys in last year's state and special elections as their supporters turned out in unexpectedly large numbers.

Trump's late campaign rallies in states he won comfortably in 2024 have encouraged Democratic hopes that Republican internal surveys show trouble. But voters can still change their minds, and undecided voters could move together toward one party. Young believes polling is improving while acknowledging its enduring difficulty: "Humans are not easy."