Americans vote in midterm elections on Tuesday. All 435 seats in the House of Representatives and 35 of the 100 seats in the Senate are contested. President Joe Biden’s Democratic Party is projected to lose its majority in at least one and possibly both chambers of Congress.

This guide explains how the elections work, why Republicans are up in the polls, why Democrats may yet defend their majority in the Senate, and which states will decide the outcome.

How the elections work

All 435 seats in the House of Representatives are contested every two years. This year’s election is the first since congressional districts were redrawn according to the 2020 census. (The census is taken every ten years.) As a result, several (new) districts don’t have an incumbent and others have multiple incumbents.

Senators are elected to six-year terms, with a third up for reelection every two years. In addition to this year’s 34 regular elections, a special Senate election is held in Oklahoma, where Republican incumbent Jim Inhofe is retiring four years early.

In addition to elections for the 118th Congress, gubernatorial, legislative and mayoral elections are held across the United States.

Most states use first-past-the-post voting rules: the candidate with the most votes wins, even if they don’t have a majority. The exceptions are Alaska and Maine, which used ranked voting: if no candidate has majority support, the candidate who places last is eliminated and their voters’ second choices are counted. The process is repeated until a candidate has at least 50 percent of the votes plus one.

In every state but North Dakota, Americans must register to vote. Nineteen states and the District of Columbia allow same-day voter registration.

Parties

• Democrats are the party of the left. Called “liberal” in the United States, they would be social democrats in Europe. The party has few social conservatives, but is home to some neoliberals and socialists.
• Republicans have moved so far to the right that they now have more in common with the European far right than mainstream Christian democratic and conservative parties.

Projections

Democrats have a majority of 220 out of 435 seats in the House. (They won 222 seats in 2020, but two Democrats have resigned.)

Republicans have fifty seats in the Senate, Democrats 48, but two independent senators — Angus King of Maine and Bernie Sanders of Vermont — caucus with the Democrats, giving Vice President Kamala Harris the tie-breaking vote. Of the 35 Senate seats that are contested, 21 are held by Republicans and fourteen by Democrats.

FiveThirtyEight, which aggregates national as well as state polls, gives Republicans an eight-out-of-ten chance of winning a majority in the House of Representatives and both parties a fifty-fifty chance of winning a majority in the Senate.

Working in Republicans’ favor

Americans tend to punish the president’s party in midterm elections. The most recent exception was 2002, when Republicans benefited from President George W. Bush’s popularity in the wake of the September 11, 2001 terrorist attacks.

Also working in Republicans’ favor are:

• High inflation — 8.2 percent in September — and the threat of a recession.
• Three in four voters worry about the increase in violent crime.
• A suspicion in Middle America that Democrats are more interested in abolishing gendered bathrooms and defunding the police (few are) than preventing recession and fighting crime.
• The fresh memory of America’s undignified retreat from Afghanistan.
• Disinterest in the revelations of the House select committee about former president Donald Trump’s involvement in the January 6, 2021 attack on the Capitol.

Working in Democrats’ favor

Democrats passed major legislations in the last two years:

• $1.9 trillion American Rescue Plan — more than twice the size of Barack Obama’s 2009 stimulus — which included an eviction moratorium, mandatory and paid sick leave for COVID-19 patients, and an increase in the minimum wage to $15 per hour.
• Infrastructure Investment and Jobs Act, which doubles federal infrastructure spending over five years.
• Inflation Reduction Act, which invests in electric cars and renewable energy, lowers health-care costs for seniors and raises taxes on corporate profits, stock buybacks and methane pollution.
• CHIPS and Science Act, which lowers taxes for semiconductor manufacturing and doubles federal spending on high-tech research and development.
• Affordable Connectivity Program, which aims to provide high-speed internet to 48 million low-income households.
• Gun control that expands background checks to include mental health.

Democrats are also helped by:

• The Supreme Court’s reversal of Roe v. Wade, the 1973 decision that legalized abortion nationwide. Only a third of Americans supported the reversal, which was the culmination of a decades-long Republican effort to stack the court with anti-abortion conservatives.
• Republicans’ nomination of far-right, pro-Trump candidates, who are unpalatable to middle-of-the-road voters.

Swing states

Senate elections are neck and neck in:

Georgia: Republican Herschel Walker was polling in first place until it emerged the anti-abortion former football player paid for the abortion of an ex-girlfriend. Despite additional allegations, of more abortions and illegitimate children, the Democratic incumbent and pastor Raphael Warnock has lost support in recent weeks. Georgia is the only state that requires a runoff if no candidate wins an outright majority.

Pennsylvania: Republicans polled well in this perennial swing state until they nominated the seemingly out-of-touch, and literally out-of-state, TV doctor Mehmet Oz, who lives in New Jersey. But then voters saw the effect a stroke had had on the Democratic candidate, Lieutenant Governor John Fetterman, and the race became a dead heat.

Nevada: The state’s former attorney general, and Republican candidate, Adam Laxalt has pulled ahead in recent polls even though most Nevadans don’t share his strong opposition to abortion, gun control and federal environmental and labor protections. The incumbent, Catherine Cortez Masto, is not well known in the state, which is majority Democratic but tends to see lower turnout from Democratic voters in midterm elections.

Arizona: Democratic incumbent Mark Kelly, a retired astronaut, has lost ground in the polls to far-right Republican candidate Blake Masters, but he remains ahead.

New Hampshire: One poll has put Republican candidate Donald Bolduc, a former brigadier general in the United States Army who argues the 2020 election was stolen from Trump, ahead of the Democratic incumbent, Maggie Hassan.

North Carolina: Ted Budd, a by-the-book Republican congressman, has been ahead of the Democratic candidate, and former chief justice of the state’s supreme court, Cheri Beasley, in every poll, but always within the margin of error.

Wisconsin: Republican incumbent Ron Johnson, another 2020 election conspiracy theorists and a global warming denier to boot, appeared to be punished by voters for his extremism but rebounded in the polls by attacking his Democratic opponent, Lieutenant Governor Mandela Barnes, for being soft on crime. If Barnes does prevail, he would become Wisconsin’s first-ever black senator.

Ohio: Author J.D. Vance, who once called Trump “reprehensible,” has done better with Republican voters since he accepted the former president’s lies. Congressman Tim Ryan, who tried for the Democratic Party’s presidential nomination in 2020, has gone down in the polls.

Possible outcomes

• Republicans win the House, Democrats keep the Senate: Biden won’t be able to legislate, but neither are Republicans able to repeal the reforms of his first two years. Republicans shut down the House investigation into the January 6 attack on the Capitol and launch their own investigations, for example into the FBI’s raid of the former president’s Mar-o-Lago estate to retrieve classified documents. Biden prioritizes foreign and trade policy, where the president has autonomy.
• Republicans win both chambers: In addition to the above, Republicans can block diplomatic and judicial appointments as well as trade deals, which require Senate approval. Attempting to repeal legislation would be pointless: Biden can still veto. But Republicans can leverage budget negotiations to, for example, lower business taxes and reduce green subsidies.
• Democrats keep both chambers: Expect legislation to legalize abortion nationwide and protect voting rights as well as more emphasis on renewable energy, including in permitting reform.

Thanks to reader J.M. van Tol for reminding me about states that allow same-day voter registration. This has been added to the story.