What are the US midterm elections and who’s running?

American elections are like Olympics. Presidential votes happen every four years in step with the summer Games: 2020, 2016, 2012, 2008, 2004 and so on.

Midterms, meanwhile, happen every four years in line with the winter Games: 2022, 2018, 2014, 2010, 2006, 2002 and so on.

Related: US midterms 2022: the key races

“Midterms” is therefore an apt name because they take place halfway through a president’s term of office. They are often regarded by pundits as a referendum on the incumbent president.

Indeed, this year’s midterms on 8 November will be voters’ the first opportunity to render a national verdict on the presidency of Joe Biden. But his name will not be on the ballot and other factors can come into play such as specific candidates, local dynamics or national issues.

Who’s running?

The constitution stipulates that every member of the House of Representatives and about a third of the Senate run for office halfway between presidential elections. The midterms typically draw a lower turnout than the race for the White House but the stakes are always high and this November will be no exception.

Up for grabs are:

• All 435 seats in the House of Representatives.

• Thirty-five seats in the 100-member Senate. This is made up of the standard 34 seats plus a special election to fill the four years remaining in the term of retiring senator James Inhofe of Oklahoma.

• Thirty-six state governorships, and three US territory governorships.

• Numerous city mayorships and local officials.

• One hundred and twenty-nine ballot measures in 36 states including laws on abortion in California, Kentucky, Michigan, Montana and Vermont.

The Senate and House make up the upper and lower chambers of Congress, which is the legislative branch of the federal government and responsible for making laws. House members serve for two years, and senators – whose terms are staggered – for six.

Each state is represented by two senators, making a total of 100. The Senate is currently evenly split between 50 Democrats (including independents Angus King and Bernie Sanders, who caucus, or align, with Democrats) and 50 Republicans. Kamala Harris, a Democrat, has cast 26 tie-breaking votes, more than any vice-president since John Calhoun in 1825-32.

Each state receives representation in the House in proportion to the size of its population but is entitled to at least one representative. The House currently has 221 Democrats, 212 Republicans and two vacancies (due to the death of Jackie Walorski and resignation of Charlie Crist).

Related: Abortion on the ballot: here are the US states voting on a woman’s right to choose

Most seats are not considered competitive due to years of redistricting and intensifying partisanship. A CBS News analysis of congressional maps found just 81 “competitive seats”, which tend to be won by one party or the other by five percentage points or less.

Historically, the sitting president’s party nearly always loses House seats in the midterms. Since the second world war the president’s party has on average lost 29 seats House seats in each president’s first midterm election, according to the Council on Foreign Relations. A notable exception was George W Bush who, following the September 11 terrorist attacks, saw Republicans gain eight House seats in 2002.

Republicans remain confident the usual pattern will hold and are focusing their campaign message on border security, crime and inflation. But there are some unusual variables this time: the spectre of Donald Trump, extreme Republican candidates and a supreme court decision to overturn the constitutional right to abortion could galvanise Democratic turnout.

Democrats feel greater confidence about holding on to the Senate. They are defending just 14 Senate seats where Republicans are defending 21.

Voters in a recent NBC News poll were split over which party they would prefer to see in charge of Congress, with 46% each backing Democrats and Republicans. Whatever happens, Joe Biden will still be president. But the results could have a profound impact on the rest of his tenure.

If Republicans do win the House or Senate or both, they will be well positioned to block much of Biden’s legislative agenda and strive to turn him into a “lame duck” president. They could launch congressional investigations into Biden’s son Hunter and other partisan targets.

With a Senate majority, Republicans would also be able to block cabinet and judicial appointments, for example if a supreme court vacancy should arise.

The midterms have an extra plot twist this time. Elections for governor, secretary of state, attorney general and state legislatures are arguably more important than ever. The winners could also have huge sway over abortion rights, voting rights and challenges to the legitimacy of future presidential elections.

Furthermore, in most states the state legislatures control the process of redrawing the boundaries of congressional districts to adjust for population changes. The party in control tends to draw boundaries to its own political advantage – known as “gerrymandering” – with far-reaching implications for future control of the House.

Another issue to watch is how the election themselves are run. Nineteen states enacted legislation in 2021 that toughened voter registration rules and curtailed or ended procedures such as mail-in and drop-off voting. The midterms will offer some of the first evidence of the consequences for democracy.

One thing is certain in America’s restless political cycle: before the last vote of the midterms is even counted, the commentary, campaigning and chaos of the next presidential election will be under way.