US election 2024: What are the House and Senate, and why do they matter?

Words by Angus Neale, ITV News Assistant Producer
When polls close across the US on November 5, the focus will be on who will be living in the White House for the next four years.
But the president’s ability to govern will be decided by the 33 Senate and 435 House races also taking place on election night.
Congress acts as a check on the president’s power. Whoever is in the Oval Office must work with senators and representatives to pass laws.
This is easy when they hold a majority in both chambers, but will that happen tomorrow?
What are the House and Senate?
For the president to pass laws, they must win majority votes in both the Senate and House.
There are 100 senators in total, with two representing each state. Members of the House, known as representatives, are elected by much smaller geographical areas called congressional districts. There are 435 districts, each electing one member.
Senate Democrats, with the support of independent senators, currently hold a slim 51-49 seat majority. In the House, the Republicans have a more convincing 221-214 seat majority.
President Biden started his presidency with control of both chambers, but the balance of power shifted after the 2022 midterm elections. This is a useful reference point, with 362 laws enacted before the change and only 106 since.
Joe Biden and Kamala Harris have criticised the Republicans for blocking their bipartisan border bill which was tabled this year. It also took months of negotiation to push forward $95 billion (£72.6 billion) worth of aid to Ukraine.
What could change?
With 49 seats, the Republicans only need to gain two more for a majority.
Ties in the Senate are broken by the vice president.
This means that if Trump wins the presidency, the GOP only needs one win for control. The current House majority is a historically small 2.06% and a 90-year low. Both races have rarely been so close.
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Voters will often pick a president aligned to one party but senators and representatives from different parties. This is known as splitting the ticket.
The congressional districts which elect representatives are small which means they better reflect the different voters across the state.
For example, California which is solidly Democratic in a presidential election, has 12 Republican representatives.
The Senate
Joe Manchin is senator for West Virginia, a state that votes solidly Republican, but he described himself as a “centrist, moderate conservative Democrat” in an interview with the local TV station WSAZ.
Manchin is not contesting the seat again, and it is likely that Jim Justice, the Republican candidate and current governor, will win.
This, along with the presidential election, could be enough to shift the balance of power.
According to the non-partisan Cook Political Report, which independently rates Senate races, there are five democratically held seats which are either a toss-up or lean Republican.
The House
The House race is even harder to predict, with every seat up for re-election. Democrats only need to turn over a dozen seats to regain control.
Polls suggest the race is a toss-up, though that doesn’t mean the majority is likely to be single digits. If a party does well with a specific group of voters, it will likely be reflected across the nation.
The Cook Political Report rates 26 house races as toss-ups. Many of these are in the blue states of New York and California. These will be fought on a combination of nationwide and local issues.
Southern California is home to several Republican-held seats near Los Angeles – these offer a significant opportunity for the Democrats.
In a poll by CSU Long Beach, the University of Southern California and Cal Poly Pomona, Democratic candidates lead in four of the six most competitive congressional districts in California.
New York is also home to a number of vulnerable Republican seats with the Cook Political Report rating three as a toss-up.
New York’s 17th Congressional District was won by republican Michael Lawler by only 0.64% of the vote.
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