Midterm math: How red maps and unpopularity shape congressional control

Election Day arrives tomorrow, bringing high stakes for control of Congress. Democrats hope to gain seats, but the path is complicated by polling gaps and a Senate map heavily favoring Republicans. President Donald Trump and his party appear confident in the final days, even as Democrats worry that polls may overestimate their strength.

A Washington Post poll released Friday in Kansas illustrated a key concern for Democrats. The survey suggested that polls in red states might systematically underestimate Republicans because they respond less often to surveys. This nonresponse bias caused errors in the 2016 and 2020 presidential elections. While it has not been as prominent in recent midterms, it remains a risk. Democrats are chasing gains in red territory, so such errors would reduce their expected wins.

The Senate map itself presents a mathematical hurdle. Even if the 2024 vote margins shifted uniformly by 10 points toward Democrats in every state, the party would still secure only 49 seats. This means Democrats could achieve one of the largest popular vote victories in decades and still fail to win the chamber. They need a net gain of four seats, which requires flipping at least two states that Trump won by double digits. Democrats are on the offensive in only one state where Kamala Harris won in 2024.

Republicans could also benefit from an upset in Maine or Michigan. Democrats have not yet secured those seats, partly because their Senate nominees are trailing the gubernatorial candidates in those states. Additionally, the GOP’s mid-decade gerrymandering effort may have created as many as nine safer Republican seats in the House. Democrats lacked the state government control to counter this move, raising the bar for their own gains.

Voter sentiment regarding the Iran war and corruption issues may also factor in. While Trump’s actions are unpopular by large margins, voters might not prioritize these issues over culture wars. The Iran conflict is not a conventional ground war with high U.S. troop deaths, which may keep it less prominent in voters’ minds.

Conversely, Democrats could capitalize on Trump’s historic unpopularity. The CNN Poll of Polls shows him 30 points underwater, with 34% approving and 64% disapproving. His coalition, which relied on young men, Latinos, and infrequent voters in 2024, has shifted sharply against him. His tariffs and the Iran war have also damaged his standing with white voters without college degrees.

Affordability remains a central issue where Trump has neglected voters’ chief concern. A New York Federal Reserve study found that consumer goods costs would have fallen without his tariffs. Trump called affordability a hoax, yet three-quarters of Americans said his administration had not focused enough on it. More Americans now blame Trump for economic problems than ever blamed Biden.

Corruption has emerged as a major concern for Republicans specifically. CNN and New York Times/Siena polls show Democrats leading Republicans on the issue by 7 and 11 points, respectively. A CBS News poll found that 52% of Americans believe corruption would increase if Republicans win the midterms, compared to 37% who said the same if Democrats won. Latino and Black voters, mobilized by deportation efforts and changes to voting rights, could play a major role in Senate races in Florida, Mississippi, Ohio, and South Carolina.