EXPLAINEROnly twice in the past 20 midterms has the president’s party gained House seats. Polls suggest 2026 won’t be the third.On Tuesday, Americans head to the polls for the United States midterm elections, held halfway through a president’s four-year term.

Voters will choose all 435 members of the US House of Representatives and 35 of the Senate’s 100 seats. The voting will decide which party controls Congress for the next two years.Although the president’s name won’t appear on the ballot, the vote is widely seen as a verdict on the presidency, and the president’s party almost always pays the price.In 18 of the 20 midterms held from 1946 to 2022, voters took House seats away from the party in control of the White House.

The average loss was 26 seats. Only Bill Clinton’s Democrats in 1998 and George W Bush’s Republicans in 2002 bucked the trend.Political scientists call this the “presidential penalty”, or the “midterm penalty”.

The president’s party tends to lose seats simply because it holds power as voters use the midterms to put a check on the White House.The House: 18 losses in 20 midtermsThe chart below shows how many House seats the president’s party has won or lost in each midterm since 1946 from Harry S Truman to Joe Biden.(Al Jazeera)Democratic presidents have suffered the biggest midterm defeats since 1946. On average, their party has lost about 29 House seats in a midterm, compared with about 23 under Republican presidents.The largest loss came in 2010, two years after Barack Obama became the first African American elected president, when Democrats lost 64 House seats and Republicans took control of the chamber.

The Democrats under Truman lost 56 seats in 1946, and under Clinton, they lost 54 in 1994, each time losing control of the House.The worst Republican defeats came under Dwight D Eisenhower when the party lost 50 seats in 1958 and Gerald Ford when it lost 48 in 1974. Donald Trump’s first midterm in 2018 cost Republicans 42 seats and control of the House.Big losses often cost the president’s party its majority.

In seven of the 20 midterms since 1946 (1946, 1954, 1994, 2006, 2010, 2018 and 2022), the House changed hands.The Senate: 13 losses in 20 midtermsThe president’s party has lost Senate seats in 13 of the 20 midterms since 1946 but lost control of the chamber in only six: 1946, 1954, 1986, 1994, 2006 and 2014.Unlike the House of Representatives, where every seat is up for election every two years, only about a third of the Senate’s 100 seats are contested in each cycle because senators serve six-year terms and their elections are staggered. That means which states are voting can matter as much as the national mood.Eisenhower’s Republicans suffered the biggest Senate loss in the period, giving up 13 seats in 1958.

Truman’s Democrats lost 12 in 1946, and Obama’s Democrats lost nine in 2014, handing control of the chamber to Republicans.At times, the two chambers have moved in opposite directions. In 1982, Ronald Reagan’s Republicans lost 26 House seats but gained one in the Senate.

In 2018, Trump’s Republicans lost the House but added a Senate seat, and in 2022, Biden’s Democrats lost the House while gaining one seat in the Senate.These split results show that a backlash against the president does not land evenly across Congress. A party can hold its ground on a favourable Senate map while losing competitive House districts.(Al Jazeera)What does it mean to lose the House or Senate?Losing seats and losing control are not the same thing.

Lyndon Johnson’s Democrats lost 47 House seats in 1966 but kept their majority. Twenty years later, Reagan’s Republicans lost only four House seats but lost the Senate after an eight-seat setback.Control of the chambers decides who sets the agenda and passes legislation.

A bill must pass both chambers before it reaches the president for his approval or veto, so an opposition majority in either can block the president’s plans.A House majority can also mean a party is able to launch investigations and impeach a president while a Senate majority can reject the president’s nominees, including federal judges, who serve for life.This year, Democrats need to gain three seats to win the House and four to win the Senate. A 50-50 split in the Senate would leave Republicans in control because Vice President JD Vance may cast tie-breaking votes.(Al Jazeera)