Democrats Eye Midterm Gains, But Polling Misses Make Victory Far From Certain

A New York Times poll puts the Senate race in South Carolina at a tie. Donald Trump won that state by 18 percentage points in 2024. Recent CNN surveys show Democrats ahead in Texas, Kansas and Maine. These are the numbers behind the Democrats' high hopes going into Tuesday's midterm elections, and they come with a long list of caveats.

The case for a good night is straightforward. The Republican president is historically unpopular, many Americans are unhappy about the rising cost of food and bills, and months of polling have been favourable. Democratic candidates lead in well over the 218 House seats needed to flip control of the lower chamber, according to the polls, and they are tied or narrowly ahead in the key Senate races.

But recent American political history gives pollsters reasons for humility. Trump outperformed predictions on his way to White House wins in 2016 and 2024, and his narrow loss in 2020 came after most surveys had suggested a more comfortable victory for Joe Biden. This year's primaries also produced surprises. Polls had Abdul El-Sayed heading for a double-digit win in Michigan's Democratic Senate primary, and the margin turned out to be 1%. In Wisconsin's Democratic primary for governor, a candidate whose polling lead approached 20 points lost by a fraction of a percent.

Clifford Young, president of Ipsos Public Affairs, says polling has grown harder because Americans have abandoned landlines and screen their calls, and because many of those who are reached are less willing to take part. He argues the problem is sharpest on the right: conservatives, especially on the far right, are anti-establishment by nature and don't participate in anything. If Republican voters are undercounted in a systematic way and pollsters don't correct for it in their turnout models, the data can tilt toward Democrats. Another explanation for Trump's earlier overperformance is that his supporters, many of them irregular voters, turned out in greater numbers than expected. Most reputable polls were off by only a few points, but in close races a few points decide the outcome, and many of this year's key Senate contests sit within that range.

There are reasons not to over-read the primary misses. Midterm general election polls have been more accurate in recent years, partly because Trump was not on the ballot. In 2022 they suggested Democrats would avoid a wipe-out despite Biden's low approval, and that is what happened: the party lost the House by slim margins but kept the Senate. Young notes that primaries have always been volatile because voters tend to decide late, so they are not evidence that the general election polls are wrong.

History says the president's party usually has a bad midterm night. In 2010, Republicans flipped more than 60 House seats and gained in the Senate, amid a sluggish recovery from the financial crash and conservative fears about Barack Obama's healthcare reforms. Polls predicted a defeat for Democrats but not on that scale. The roles are now reversed, with the economy again on voters' minds.

Still, there are reasons the Democrats' night may fall short of expectations. Much of the contested territory is ground Trump won easily just two years ago, making it something like a home game for Republicans. Changes to House boundaries over the years mean fewer seats switch parties. Deeper polarisation has left fewer swing voters, and some experts call the electorate calcified. Republicans have also enjoyed a cash advantage, giving them hundreds of millions of dollars for attack ads in the final weeks.

Polling averages are generally more reliable than individual surveys, which carry a built-in margin of error. Even the kind of Republican lean seen in recent presidential elections would still leave Democrats likely to have a good night on 3 November. Errors can also run the other way: in last year's state and special elections, Democrats beat most surveys because their voters turned out in unexpectedly large numbers. Trump's rallies in the closing days, in states he won comfortably in 2024, may signal that Republican internal polling shows trouble.

The final variable is late deciders. Undecided voters remain a key segment in many of these races, and sometimes they break the same way. Young believes pollsters are doing a better job, but says the difficulty isn't only with the pollsters: they are dealing with humans, and humans are not easy.