The Democratic Party holds high expectations ahead of Tuesday’s US midterms, buoyed by months of favourable polling and public dissatisfaction with rising food and utility costs. The Republican president is historically unpopular, which further benefits the opposition.
According to polls, Democratic candidates are leading in significantly more than 218 seats, the number required to seize control of the House of Representatives. In key Senate races, Democrats are tied or narrowly ahead. A recent New York Times poll showed a tie in the South Carolina Senate race, where Donald Trump won by 18 percentage points in 2024. CNN polls indicated that Democrats are leading in Texas, Kansas and Maine.
However, history advises caution. Trump outperformed predictions on his way to victory in the White House in 2016 and 2024, and his narrow defeat in 2020 followed a period when most polls showed a more comfortable win for Democrat Joe Biden. Earlier this year, polls suggested Democrat Abdul El-Sayed was on track for a double-digit victory in the Democratic Senate primary in Michigan, but the result was 1 per cent. In the Democratic primary for governor of Wisconsin, a candidate who was leading by nearly 20 points lost by a fraction of a percentage point.
Clifford Young, president of Ipsos Public Affairs, notes that polling has become difficult as it is increasingly hard to reach Americans. Citizens are abandoning landline phones and filtering calls, while many contacted individuals have become less willing to participate. Conservatives, particularly the far right, are inherently anti-establishment, participate in nothing and hate the system. If Republican voters are systematically undercounted and polls do not account for this, the data can be skewed towards Democrats. One explanation for Trump’s outperformance is that his supporters, many of whom were not regular voters, turned out in greater numbers than expected.
Unpredictability is a major factor in recent significant polling misses. Margins in most reputable polls were off by only a few percentage points, but in a tight race, those few points can mean the difference between victory and defeat. This year, many key Senate races are within those narrow margins. In recent years, polls for general elections in midterms have proven more accurate. Trump was not on the ballot in 2022, when polls suggested Democrats would defy conventional expectations and avoid an electoral defeat despite Biden’s low approval ratings. Democrats lost control of the House that year, but the margins were tight, while they retained control of the Senate.
This year’s primary results have brought greater surprises. Young says that nomination elections have historically been volatile and that voters tend to decide at the last minute. It is customary for the president’s party to have a bad night in midterms. One of the most significant examples was 2010, when Republicans flipped more than 60 seats in the House and made gains in the Senate. This followed an economy struggling to recover from the financial crisis and conservative fears of President Barack Obama’s healthcare reforms. The 2010 defeat was predicted by polls, but not quite on that scale. This time the roles are reversed, with Democrats in opposition and the economy on voters’ minds.
Much of the territory at stake in these elections was easily won by Trump in the presidential election just two years ago, making it a home game for Republicans. Over the years, there have been changes to House electoral boundaries, meaning fewer seats swing between parties. Due to deeper polarisation in the country, there are fewer undecided voters, and some experts describe the electorate as ossified. Republicans have enjoyed a cash advantage, having hundreds of millions of dollars to spend on attack ads in the final weeks.
Polling models attempt to account for shifting bases. A well-designed poll is built to predict problems that might make it less reliable. Individual polls have a built-in margin of error, a statistical level of uncertainty based on sample size relative to the total population. Taken as an aggregate, polling averages, rather than individual surveys, can provide a more accurate prediction. Currently, even the type of polling bias seen in recent presidential elections would suggest that Democrats are likely to have a good night on 3 November. However, polling inaccuracies can go in both directions. In last year’s state and special elections, Democrats outperformed most polls, and Democratic voters turned out in unexpectedly high numbers.
