Democrats have high hopes for midterms, but polls have let them down before

Democrats are entering the midterms with big expectations, but polls have misled them before

The Democratic Party is looking ahead to these crucial US midterm elections with high hopes.

A historically unpopular Republican president, an American public frustrated by rising food and bill costs, and months of favourable polling all suggest a strong night for them on Tuesday.

Polls indicate Democratic candidates are ahead in far more than the 218 seats required to take control of the lower chamber of Congress, the House of Representatives. They are also tied or slightly ahead in the major Senate contests.

A recent New York Times poll showed the Senate race in South Carolina, a state Donald Trump won by 18 percentage points in 2024, as tied. Recent CNN polls have Democrats leading in Texas, Kansas and Maine.

But can the polls be trusted?

Recent US political history is full of examples in which public opinion surveys, even those conducted just days before an election, missed the mark. Trump beat expectations on his way to White House wins in 2016 and 2024. His narrow loss in 2020 followed most polls showing a more comfortable victory for Democrat Joe Biden.

Earlier this year, polls pointed to a double-digit win for Democrat Abdul El-Sayed in Michigan's Democratic Senate primary. Instead, he won by 1%. In Wisconsin's Democratic primary for governor, the candidate who had been leading by nearly 20 points lost by less than a percentage point.

Clifford Young, president of Ipsos Public Affairs, which carries out political and market research surveys, says polling has become harder in recent years because it is more difficult to reach Americans as they drop land lines and ignore calls. Many of the people pollsters do reach are also less willing to take part.

"Conservatives, especially those on the far right, are much more anti-establishment by nature," Young says. "And they don't participate in anything. They hate the system. That's the challenge."

If Republican voters are being systematically undercounted, and polling firms fail to account for that when building their voting models, the data could tilt toward Democrats.

That is one reason it has become harder for pollsters to forecast what the electorate will look like.

One reason Trump outperformed expectations in earlier presidential elections is that his supporters — many of them not habitual voters — turned out in greater numbers than expected.

That unpredictability is a major factor behind those recent polling misses, even when the margins in most respected polls were only off by a few points.

In a tight race, those few points can decide whether a candidate wins or loses. This year, many of the key Senate races fall within those narrow margins.

In recent years, midterm general election polls have tended to be more accurate, partly because Trump — and his unusual ability to bring disaffected Americans to the polls — was not on the ballot.

In 2022, they indicated Democrats would defy expectations and avoid a wipeout despite Biden's weak approval ratings.

Although the party lost the House of Representatives, the margins were narrow and Democrats kept control of the US Senate.

While this year's primary results produced bigger surprises, Young says primary contests have historically been volatile, with voters more likely to decide at the last minute. He says that should not be taken as evidence that the current general election polls are wrong.

It is normal for the president's party to have a poor midterm night.

One of the clearest recent examples came in 2010, when Republicans took more than 60 House seats and gained ground in the Senate.

That happened against the backdrop of an economy still struggling to recover from the financial crash and conservative concern over President Barack Obama's healthcare reforms. Polls had predicted a defeat, but not on that scale.

This time the roles are reversed, with Democrats in opposition and the economy on voters' minds. But there are several reasons Democrats may not do as well as history might imply, or as some might expect given Trump's low approval rating.

First, much of the territory in play in this election was won easily by Trump in the presidential election just two years ago. It is like a home game for Republicans.

Second, House district boundaries have changed over the years in ways that have reduced the number of seats that switch hands between the parties.

Third, deeper political polarization has left fewer swing voters. Some experts describe the electorate as calcified.

Republicans also have a financial edge, giving them hundreds of millions of dollars to spend on attack ads in the final weeks.

Polling models are designed to account for changing fundamentals. A well-constructed poll is meant to anticipate factors that could make it less reliable.

Each poll has a built-in margin of error — a statistical measure of uncertainty based on sample size compared with the overall population. Taken together, polling averages rather than individual surveys can offer a more accurate forecast.

Right now, even the kind of polling advantage seen in recent presidential elections would suggest Democrats are likely to have a good night on 3 November. And polling errors can cut both ways. 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 final days of the campaign in states he won comfortably in 2024 will have boosted Democrats' hopes that Republican internal polling shows conservative candidates in trouble.

Still, voters could change their minds right up to election day.

Sometimes late shifts change the shape of a race. Sometimes undecided voters, who still make up an important share of the electorate in many of these contests, all break the same way.

In polling, uncertainty will always remain — and managing it is the art that goes with the science.

"I think we're doing a better job," says Young. "But part of the problem we always have is not just with the pollsters. Part of it is that we're dealing with humans. Humans are not easy."