How the balance of power in US Senate could change after tomorrow's midterms

The US Senate is the upper chamber on Capitol Hill. There are 100 Senators, two from each state, and Republicans currently hold a razor thin majority with 51 seats.

The US Senate writes and passes laws but has a number of other powers and responsibilities, from ratifying treaties with other countries to overseeing investigations of officials and public bodies.

Senators have six-year terms and just 35 seats are up for re-election. Most of these are currently held by Democrats, making it hard for them to make gains. 

The president's party could increase their hold of the Senate, since 10 of the Democratic seats are in states carried by Donald Trump in 2016 - five of which he won by double digits. Mr Trump has begun an intensive campaign tour designed to make the midterms a referendum on his presidency in the hope of maintaining swing seats. 

While the US economy is booming, Mr Trump's approval rating is hovering around 40 per cent - a worrying sign for Republicans.

The Democrats are holding out hope that a "blue wave" of supporters energised by opposition to the Trump presidency can lead to a string of unexpected victories.

The powers and responsibilities of the US Senate

The Senate and the House of Representatives are considered equal but different partners in the legislative branch of government. Their most important power is the authority to make laws.

They also share the ability to declare war,  raise an army and establish rules of immigration and naturalisation,  among others.

Each also has unique responsibilities. In the case of the Senate, these include confirming by majority vote the appointment of federal judges, ambassadors and Cabinet posts and approving (by a two thirds vote) treaties with other nations.

And while it is the House that votes to impeach an elected official, it is the Senate where the hearing is conducted. A two-thirds majority is needed for conviction and removal from office.

Why a Senate term is longer than in the House

Congress was established in article one of the US constitution, part of the great “American experiment” as the nation’s founding fathers tried to build a country and democracy almost from scratch.

It set up a bicameral structure with states electing representatives proportionate to population to a lower house plus two senators regardless of size to the upper chamber, (protecting the interests of smaller states).

Representatives serve two-year terms and face re-election every two years. The short span and frequent elections are meant to keep them closely aligned with public will.

However, the founders also feared mob rule and structured the Senate as an antidote. Its longer, six-year terms are designed to isolate members from short-lived whims and create a more contemplative atmosphere.

What would losing the Senate mean for Donald Trump?

A Democratic controlled House and Senate could launch impeachment hearings - but it would still require bipartisan support to pass, which is extremely unlikely.

A more pressing problem for Mr Trump would be the ability to have his judicial nominees approved, which is done by a vote of the full Senate.

The importance of that has been proved by the confirmation of Brett Kavanaugh to the Supreme Court. The Senate voted 50 to 48 for him to be approved - the tightest of margins. 

One major talking point in these midterm elections is the 'I' word - impeachment. It is the House of Representatives that launches articles of impeachment, but any trial would be held in the Senate and its outcome voted on by its members.

Republicans have been eager to bring up impeachment on the campaign trail - arguing a Democrat-controlled Congress would attempt to impeach Mr Trump. Democratic leaders have largely played down the claims, aware that there is little public appetite for it. However a handful of prominent Democrats, including Maxine Waters, have suggested the president should be impeached.

Mr Trump has brought it up himself on his campaign tour, telling supporters at a rally it will be "your fault" if he gets impeached.

"You didn't go out to vote - that's the only way it could happen," the president said during a rally in Montana. "I'll be the only President in history they'll say: 'What a job he's done! By the way, we're impeaching him'.

"This election, you aren't just voting for a candidate, you are voting for which party controls Congress," he said.

Seats to watch

Nevada

Senator Dean Heller's election fight is an interesting one to watch. He is the only Republican senator up for re-election in a state that Hillary Clinton won in 2016. 

Senator Heller's Democratic opponent, Congresswoman Jacky Rosen, has support from anti-Trump female voters. She is also hoping Nevada's growing Hispanic population will help her to victory in November.

However she faces an uphill battle in encouraging voter turnout, and Republicans are relying on white rural voters to come out to support Mr Heller.

North Dakota

Senator Heidi Heitkamp, who is facing re-election in a state Mr Trump won by nearly 40 points in 2016, is considered the most endangered Democrat in the Senate.

Ms Heitkamp faces pressure from conservative voters after voting against Brett Kavanaugh's Supreme Court nomination, a move she admitted was not a "smart political vote". However, Democrat donors have injected around $13 million into her campaign since her no vote.

Her opponent, Kevin Crammer, has the backing of the president. Mr Trump headlined a fundraiser for the Republican in early September which brought in more than $1 million in donations to his campaign.

Florida

The race between Democratic Senator Bill Nelson and his Republican challenger Rick Scott is one of the most expensive of the year. Mr Scott, Florida's governor, has challenged Mr Nelson's record in Washington and distanced himself from the president so as not to lose out on Puerto Rican voters.

Republicans see the seat as one of their most promising chances of picking up an extra Senate seat and have spent heavily in the race. Polls show the two almost neck and neck - an interesting race to tune into on election night.

Texas

Despite being a presidential candidate in 2016, Republican Senator Ted Cruz is now fighting for his political life in Texas. His Democratic challenger  - Bete O'Rourke - has brought Mr Cruz's lead in the deeply red state down to single digits, shocking political pundits.

Mr Trump has overcome his previous animosity with the Senator to lend his support to his campaign. Donald Jnr has already been deployed to campaign for Mr Cruz and the president himself made an appearance late in October in a bid to bolster support.

Mr O'Rourke, a 45-year-old congressman, has campaigned on a platform of inclusion and optimism, particularly on issues such as immigration. It is a message that chimes with the state's growing Hispanic population, which currently stands at 39 per cent.

Pollsters still predict a Cruz victory but Mr O'Rourke's popularity and upbeat campaign rallies have left Republican operatives deeply troubled. Losing a Republican seat in Texas would be a big political upset.