One race has dominated this year's election cycle in the United States: the battle between Kamala Harris and Donald Trump to win the White House.
But on Nov. 5, Americans will also vote to fill other key positions outside the presidency. Many of those races will determine who controls Congress, an important lever of power.
One-third of the United States Senate is at stake, as is the entire House of Representatives. That is equivalent to 34 seats in the upper house of Congress and 435 in the lower house.
But why are representatives voted every two years and senators every six? And why aren't all 100 seats in the Senate contested at the same time, like in the House?
Staggered Senate races
The answer goes back to the founding of the country, when the men who wrote the United States Constitution decided to base Senate elections on a “three-class system.”
According to a Senate fact sheet, “at the beginning of the first session of Congress in 1789, senators were divided into three classes by lot and senators from the same state were assigned to separate groups.”
The first group saw its mandate expire in two years, the second in four and the third in six years.
“The subsequent elections for all classes were for a full six-year term in the Senate,” the fact sheet explains.
The concept period gives stability to American politics. As a result of the Senate's staggered voting system, two-thirds of senators remain in office each time national elections are held, once every two years.
How does this compare to the House of Representatives?
Things are different in the lower house of Congress, where its 435 members serve two-year terms and are elected in each vote.
Having to run for re-election every two years makes House members more vulnerable to losing their seats.
However, some representatives have been in the House for decades: Former House Speaker Nancy Pelosi, for example, has been in office for 37 years.
What could happen on November 5?
Currently, the Democratic Party has slight control over the Senate. The party and its independent allies hold 51 seats, compared to 49 seats for the Republicans.
To take control of the chamber, Republicans need a net gain of one Senate seat if Trump defeats Harris in the presidential race.
This is because the vice president acts as the tie-breaking vote in the Senate. If Republicans win the White House, Trump's running mate, JD Vance, would fill that seat and vote with the Republicans.
If Harris wins, giving her vice president, Tim Walz, that tie-breaking power, then Republicans would need a net gain of two Senate seats to take control.
Of the 34 Senate seats up for grabs next month, 23 are held by Democrats or independents.
According to recent polls, it likely comes down to a few Senate races that appear to be close, even in the swing state of Michigan.
Meanwhile, Republicans hope to defend their management of the House of Representatives. There are currently 220 Republicans in the House, compared to 212 Democrats, and more than a dozen races are considered tossed.