Why are only some US Senate seats up for grabs? What to know in 500 words
Americans will cast ballots to fill other important positions besides the presidency on November 5. 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 do senators get their votes every six years and representatives get theirs every two years? 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”.
Senate members were assigned to distinct groups at the start of the first session of Congress in 1789, according to a Senate factsheet.
The first group’s terms came to an end in two years, the second four, and the third six.
“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?
The lower chamber of Congress, which has 435 members who serve for two years and are elected at the end of every vote, has a different structure.
Members of the House  are more prone to losing their seats because they must run for re-election every two years.
Nevertheless, some representatives have been in the House for decades: Former House Speaker Nancy Pelosi, for example, has served for 37 years.
On November 5, what might happen?
The Senate is currently under the control of the Democratic Party. The party and allied independents hold 51 seats, compared with 49 seats held by Republicans.
If Trump defeats Harris in the presidential election, Republicans must secure a net gain of one Senate seat for the party to win control of the chamber.
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.