Why are only some US Senate seats up for grabs? What to know in 500 words 

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 November 5, Americans also will vote to fill other key posts outside the presidency. Many of those races will determine who controls Congress, an important lever of power.

One-third of the US Senate is up for grabs, as well as the entire House of Representatives. That amounts to 34 seats in Congress’s upper chamber and 435 in the lower one.

But why are representatives voted on every two years — and senators every six? And why aren’t all 100 seats in the Senate contested at once, like in the House?

Staggered races in the Senate

The answer goes back to the country’s founding, when the men who wrote the US Constitution decided to base Senate elections on a “three-class system”.

According to a Senate factsheet, “at the start of the first session of Congress in 1789, senators were divided into the three classes by lot with same-state senators assigned to separate groups”.

The first group saw their term expire in two years, the second in four and the third in six years.

“Subsequent elections to all classes were for a full six-year Senate term,” the factsheet explains.

The idea was to give stability to US politics. As a result of the Senate’s staggered voting system, two-thirds of the senators remain in their posts every time a national election happens, once every two years.

How does this compare to the House of Representatives?

Things are different in the lower chamber of Congress, where all of its 435 members serve two-year terms and are up for election at every vote.

Having to stand for re-election every two years makes members of the House more vulnerable to losing their seats.

Nevertheless, some representatives have been in the House for decades: Former House Speaker Nancy Pelosi, for example, has served for 37 years.

What could happen on November 5?

The Democratic Party currently has a thin grip on the Senate. The party and allied independents hold 51 seats, compared with 49 seats held by Republicans.

To take control of the chamber, Republicans need a net gain of one Senate seat if Trump defeats Harris in the presidential race.

That’s 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 role and vote with 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 gain control.

Of the 34 Senate seats being contested next month, 23 are held by Democrats or independents.

According to recent polls, it will likely come down to a few Senate races that appear to be neck-and-neck, including in the swing state of Michigan.

The Republicans, meanwhile, are hoping to defend their control of the House of Representatives. Currently, there are 220 Republicans in the House, compared with 212 Democrats — with more than a dozen races considered toss-ups.