Political polls regarding US elections in 2020 overstated Democratic support “across the board”, US political scientists found, while understating support for Republicans and Donald Trump.

The finding, which will alarm Democrats aiming to hold on to their narrow control of the US House and Senate in 2022, is contained in a new study by the American Association for Public Opinion Research.

Josh Clinton, a Vanderbilt University professor and AAPOR taskforce member, told the Washington Post: “There was a systematic error that was found in terms of the overstatement for Democratic support across the board.

“It didn’t matter what type of poll you were doing, whether you’re interviewing by phone or internet or whatever. And it didn’t matter what type of race, whether Trump was on the ballot or was not on the ballot.”

Polls were better at predicting support for Joe Biden against Trump in the presidential election than for Democrats in state elections, the study said.

In 2020, polling pointed to Democratic gains in the House, only for Republicans to eat into the majority which made Nancy Pelosi speaker in 2018.

Parties which hold the White House often lose seats in midterm elections. Republicans in Washington are duly bullish about their chances of retaking the House next year, boosted by GOP-run state governments implementing laws meant to restrict voting among communities likely to vote Democratic and to make it easier to overturn results.

Speaking at a conservative conference earlier this year, the House minority leader, Kevin McCarthy, said: “We’re going to get the majority back … I would bet my house.”

The AAPOR study found that polls were more accurate in predicting a popular-vote win for Biden, a contest he eventually won by more than 7m ballots.

The electoral college result was 306-232, the same margin by which Donald Trump beat Hillary Clinton in 2016, when the Republican lost the popular vote by “only” 2.8m.

On Monday, the AAPOR website was down for maintenance. As quoted by the Post, its study said: “That the polls overstated Biden’s support more in whiter, more rural, and less densely populated states is suggestive (but not conclusive) that the polling error resulted from too few Trump supporters responding to polls.

“A larger polling error was found in states with more Trump supporters.”

Josh Clinton, the Vanderbilt professor, said: “It’s possible that if President Trump is no longer on the ticket or if it’s a midterm election where we know that the electorate differs in the presidential election, that the issue will kind of self-resolve itself.

“But if the polls do well in 2022, then we don’t know if the issue is solved or whether it’s just a phenomenon that’s unique to presidential elections, with particular candidates who are making appeals about ‘Don’t trust the news, don’t trust the polls’ that kind of results in taking polls becoming a political act.”