American voters will head to the polls in early November for critical midterm elections that will determine the makeup of the next United States Congress.

The election results will set the tone for the rest of Joe Biden’s first term as president, as analysts have said they are likely to shake up a political arena already marked by deepening partisanship and polarisation.

A number of local and statewide races will be held across the country on November 8, and US Senate and House of Representatives seats also will be up for grabs.

History suggests the midterms could spell trouble for Democrats as the party in power. But with Biden scoring recent legislative victories and liberal voters rallying behind reproductive rights, a Republican victory is not guaranteed.

Here, Al Jazeera looks at what’s at stake in the election and how the system works:

What are the midterm elections?

The midterms take place two years after a presidential election, mid-way through a US president’s four-year term.

Typically, about one-third of the seats in the 100-member US Senate are up for grabs, along with all 435 seats in the US House of Representatives.

Governorships, state legislatures, and local councils and school boards will also be contested around the country.

How many House and Senate seats will be up for grabs?

All 435 House seats will be contested in November, while 35 senators will also be elected.

How are seats in the House and Senate distributed?

Each state, regardless of its population, gets two Senate seats, while House seats are distributed to the states based on their size.

For example, California – the most populous state – has 52 House members and Wyoming – the least populous state – has one. But both states each have two senators.

When is the election?

This year, the midterms will take place on November 8.

Major US elections are held on the first Tuesday of November, unless that first Tuesday falls on the first day of the month. In that case, the vote happens a week later – on November 8.

Why are US elections held so frequently?

The main reason is that House terms are only two years.

Senate terms are six years, so whether it’s a presidential or midterm election year, 33 to 34 Senate seats are typically up for grabs every two years, too.

Who currently controls Congress and what does that mean?

The Democratic Party currently holds slim majorities in both the House and the Senate, giving them a governing trifecta with Biden in the White House.

For a bill to pass, it needs the approval of both chambers – so control of Congress is critical to get legislation passed in the United States.

In addition to legislating, the Senate is tasked with confirming the president’s judicial and cabinet nominees, as well as approving formal treaties.

Is President Biden’s name going to be on the ballot?

In US politics, the president is elected for a four-year term and is not up for re-election in the midterms. But while Biden’s name will not be on any ballots, his administration’s perceived successes – and failures – can still affect the results.

“The midterms are, at least partly, a referendum on the performance of the current president,” said Alan Abramowitz, a professor of political science at Emory University in Atlanta.

Are Democrats expected to retain control of Congress?

Abramowitz told Al Jazeera that “there is a very consistent historical trend that the president’s party almost always loses seats in the House, and often loses seats in the Senate”.

But Democrats are hoping to buck that trend this year by rallying behind abortion rights after the US Supreme Court overturned the constitutional right to the procedure. They are also trying to paint the Republican Party as beholden to former President Donald Trump’s right-wing agenda.

“This very dramatic decision to overturn Roe v Wade is having a clear impact on the political climate,” Abramowitz said.

Is Trump playing a role in the midterms?

It is not common for a former president to play an outsized role in the midterms, but Trump is not a common political figure.

He has successfully backed dozens of candidates in Republican primaries before the November vote as he continues to tease another possible White House run in 2024.

Trump and many of the candidates he has endorsed have been promoting false claims of widespread voter fraud in the 2020 election, raising concerns among critics about their commitment to the democratic process.

“Trump has supported some candidates with very little political experience, but they support the stolen election lie,” said Abramowitz. “But that claim doesn’t really appeal to voters outside of the party’s base. There’s a growing sense that the GOP has moved very far to the right.”

What would happen if Republicans took back Congress?

Abramowitz summed up that possible outcome in one word: “gridlock”.

“There would be a lot of conflict between Congress and the White House, and the GOP would look to block any progress on Biden’s agenda,” he said. “They could run out the clock confirming any potential Supreme Court appointments, and they could block any further judicial nominations.”

Congressional investigations into Trump would come to a halt, and Republicans could go on the offensive, he predicted. “They could push investigations of their own into Biden officials or even impeachment proceedings,” said Abramowitz.

What if the GOP fails to win?

Given what are typically highly favourable conditions for the party not in the White House during a midterm year, some would see anything less than a Republican majority in both chambers as a significant failure.

“It would say a lot if the Republicans have a disappointing year,” said Abramowitz. “It would be a pretty significant indicator that voters are not happy with the direction the party is going, and it would intensify conflicts within the GOP.”

Abramowitz added that a midterm setback could lead the GOP to reconsider its relationship with Trump. “Trump maintains tremendous influence over the Republican Party, and a poor performance in 2022 could loosen his grip,” he said.

Could Democrats get an even larger majority?

If Democrats are able to win enough seats to gain a more comfortable majority in Congress, the party could pursue a more ambitious agenda. However, Abramowitz said that outcome is fairly remote.

“I would say that, at least for now, it seems likely that Republicans could pick up the House while Democrats maintain a moderate advantage to hold onto the Senate.”

This would create a situation where control of Congress is split.

What happens in case of a split?

As Abramowitz said earlier, gridlock.

Even with slim majorities in the House and Senate, the Democrats’ agenda has been severely curtailed. If control of the legislative branch were to be split, the Republican Party would have a veto over any potential legislation.

“Republicans would be able to block anything Biden wants, but wouldn’t be able to pass any of their own agenda either,” said Abramowitz.