With the midterms looming, Al Jazeera breaks down how the vote works, why it matters and what to expect.

US midterm elections are scheduled for November 6, and the vote is largely expected to be a referendum on President Donald Trump‘s performance since taking office.

Held every four years, midterm elections occur halfway through every presidential term. Midterms generally garner low voter participation, but the Pew Research Center recently published a poll that found increased enthusiasm among voters this year.

November’s vote will determine the future of Trump’s legislative agenda, with Democrats hoping to take back a majority in the US Congress. Republicans currently enjoy a majority in both chambers of Congress: the Senate and the House of Representatives.

This year all 435 House seats, 35 Senate seats and 39 state and territories governorships are up for grabs.

How do midterms work?

Every two years, all 435 House seats are up for election. But in the Senate, where elected officials serve staggered six-year terms, one-third of the seats are decided during midterms.

In the House, 218 seats are necessary for a majority, while 51 seats constitute a majority in the Senate.

Additionally, state races will decide the elected representatives for state legislatures and dozens of governorships during midterm elections. Thousands of seats in state houses and senates will also be up for grabs this year.

How many people vote?

According to the US Census Bureau, 157.6 million people reported being registered to vote in the US.

Voter turnout is generally around 40 percent during midterms, according to FairVote. But Pew’s new poll suggests that more than half of registered voters will take to the ballot box this year.

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With the country increasingly polarised by Trump, more voters could cast their votes in November.

Turnout for House primaries was nearly 20 percent, which marks a large increase compared with previous years, according to Pew. That poll found that turnout was higher for both Democrats and Republicans, but that Democrats bested their Republican counterparts, growing by 4.6 percentage points.

Why do midterms matter?

For the viability of President Trump’s policy agenda, the turnout of the midterms will be a decisive factor. If Republicans can hold a majority in Congress, Trump can expect less resistance while pushing through his policies. If Democrats can take back the Congress, they could disrupt Trump’s policy plans and stop him from implementing a number of campaign promises.

For instance, if Democrats controlled Congress, they could block his ongoing efforts to crack down on immigrants, including the building of the wall on the US-Mexico border, efforts to remove Obama-era protections for undocumented immigrants and restrict the actions of the Immigration and Customs Enforcement (ICE) agency.

After the controversy surrounding the confirmation of Supreme Court Justice Brett Kavanaugh, whose hearings focused on several allegations that he was complicit in sexual assault, Democrats see the midterms as an opportunity to prevent Trump’s future Supreme Court nominations. Without taking a majority in the Senate, the upper house of Congress, Democrats would be unable to achieve this goal.

Most importantly, a Democratic majority in Congress could potentially launch impeachment proceedings against Trump, a threat that the president and his party have emphasised to encourage Republican voters to take to the ballot box next month.

Those opposed to the confirmation of Supreme Court Justice Brett Kavanaugh vowed to move their fight to the November midterms [File: Jacquelyn Martin/AP Photo]

Meanwhile, many state legislatures play an important role in redistricting processes, healthcare access, budgetary decisions and civil rights legislation, among others.

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Gubernatorial elections could also usher in significant changes. In Florida, Governor Rick Scott has undone efforts to restore voting rights to convicted felons, at least 1.5 million of whom are disenfranchised in that state.

And whichever party takes control will wield influence over the next redistricting process, slated to take place in 2020.

What are the main issues?

The largest partisan divides between Republicans and Democrats stemmed from immigration, healthcare and the role of the US in the world, according to a Pew survey.

According to the September Pew report, about 75 percent of voters ranked healthcare as a “very important”.

Women’s rights, Supreme Court nominations and the economy also stand out as important factors in the upcoming midterms.

Additionally, the Pew survey found that race was a central issue, with 85 percent of Democrats supporting more efforts towards equality for African Americans, compared with only 29 percent of Republicans.

What can you expect?

The final outcome of the midterms will likely depend on results in several key races across the country. Some of the most important races include the election for Republican Jeff Flake’s seat. Flake is not seeking re-election this year and the current race between Republic Martha McSally and Democrat Kyrsten Sinema is too close to call, according to a number of projection websites.

Many are also keeping an eye on tight Senate races in Indiana, Florida, Missouri, Nevada, North Dakota, and West Virginia, among others.

There are also a number of key races in the House, including California’s 25th and 48th districts, Kentucky’s 6th District, Texas’s 32nd District, New York’s 19th District, Illinois’s 14th District, Iowa’s 1st District, among several others.

Last month, a record number of people – 800,000 – registered to vote on National Voter Registration Day, although it remains unclear how many will show up on voter day.

According to FiveThirtyEight’s most recent predictions, Democrats have a one-in-five chance of taking control of the Senate, while their chances of gaining a majority in the House sit at 81.3 percent.

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The website’s statistics also found that Trump’s approval rating is currently 41.8 percent, and the percentage of those who disapprove of the president is more than 52 percent.

In the increasingly unlikely scenario that Republicans lose control of both the Senate and the House, they would no longer be able to pass laws without the support of Democrats.

And if Democrats took control of the Senate, they would be able to block Trump’s nominations for judges and other positions.

But if the Republican Party maintains its grip on both or either chambers, Democrats ability to challenge Trump’s agenda will be significantly reduced.