Who controls the US Senate and can Georgia flip it?

The US Senate is the upper chamber of Congress, the lower chamber being the House of Representatives. Together they are the legislative branch of the federal government, the other two branches being the executive branch (the presidency, cabinet, federal agencies) and the judicial branch (the Supreme Court and other courts).

Consisting of 100 Senators — two per state — each lawmaker serves a six-year term staggered against the other representative from their state. Since the ratification of the 17th Amendment in 1913 they have been elected by popular vote.

At present, the political breakdown of the Senate is that the Republican Party holds a majority with 52 seats, while the Democratic Party has 46 seats, but are also joined by two independent senators who caucus with the party.

Read more: Latest Georgia Senate runoff election odds

The Republicans are led by Senate majority leader Mitch McConnell of Kentucky, and majority whip John Thune of South Dakota. As in the British parliament, party whips work to ensure senators vote in line with the desires of party leadership.

The Democrats are led by Senate minority leader Chuck Schumer of New York, and minority whip Dick Durbin of Illinois.

The two independent senators are Angus King Jr of Maine and Bernie Sanders of Vermont.

With party line voting and increasingly entrenched partisanship, Republicans have the votes to thwart any legislation passed by the Democrat-controlled House.

The runoff elections in Georgia are therefore important for several reasons. First off, they are happening on 5 January 2021 because no Senate candidate in the state achieved a majority in the 3 November general election, which is required under Georgia state law.

Having both Senate seats on the ballot in a state is a very rare occurrence. While incumbent Republican David Perdue’s term is up, fellow Republican Kelly Loeffler was appointed to her seat by Governor Brian Kemp following the resignation of her predecessor, Johnny Isakson. As such she had to face a popular vote in a special election.

They are running against Democrats Jon Ossoff and Raphael Warnock, and polling has each race close to a tie. This is important as Democrats narrowly won the state of Georgia in the presidential election for the first time since 1992. They are hoping to repeat this feat in January as the fate of President-elect Joe Biden’s legislative agenda hangs in the balance.

Read more: When will we know the results of the Georgia Senate runoff election?

There are three possible outcomes. If the Republicans win both seats and return Mr Perdue and Ms Loeffler to the Senate then the Biden administration will find it extremely difficult to pass legislation through the upper chamber.

Any bills that are passed will likely include huge concessions on the part of Democrats, and while Mr Biden prides himself on his many years of work in the Senate and ability to reach a compromise, it is far from ideal for the party, especially the progressive wing. Republicans would also retain control of what comes up for a vote — more on that later.

The same is true if each party won one seat. A split of 51:49 in favour of the Republicans would present the same challenges, but negotiating a compromise with just one senator from the opposing party is more attainable than negotiating with two.

Were the Democrats to win both seats and split the chamber 50:50 between the parties, things get interesting as the president of the Senate casts the deciding vote to break any tie on a simple majority vote — if present.

Helpfully for the Democrats, that role is fulfilled by the vice president, soon to be Kamala Harris. There would be unified government across the executive branch and both legislative chambers for the first two years of the Biden administration.

Senators that often cross party lines, such as Susan Collins of Maine for the Republicans or Joe Manchin of West Virginia for the Democrats, will be important in passing legislation.

As for leadership in a 50:50 split, the main task is setting the agenda of what gets brought up for debate or given a vote and what gets left to flounder. Currently, Mr McConnell holds that power. If Democrats are victorious in Georgia, Mr Schumer will be in control.

This would allow legislation on a number of Democrat priorities to move forward, including additional pandemic relief funds, potential tax increases on the very wealthy, and a major infrastructure package as promised in the election.

There would also be action on the John Lewis Voting Rights Act, climate change, criminal justice reform, and marijuana decriminalisation. In addition, the fate of the Affordable Care Act would be decided.

A Senate run by Mr Schumer would also likely move to get rid of the filibuster ensuring that legislation can be brought to a vote by a simple majority of 51, rather than dying a death under the threat of the need for 60 votes for cloture on efforts to prolong debate indefinitely.

Senate control is also important for cabinet and judicial nominations, which could be crucial for Mr Biden to get the team in place that he wants.

Committee chair positions would also go to Democrats and they hold the key to deciding what to bring to a vote, be it legislation or nominations.

They also determine whether to carry out investigations. Were Republicans to maintain their majority they would likely pursue probes into the 2020 election and Hunter Biden, potentially handicapping or disrupting the administration from early on.

Democrats therefore see the Georgia runoff races as vital to advancing their legislative agenda, while Republicans see Mr Perdue and Ms Loeffler as a firewall against that threat.

On 5 January, approximately 5 million voters in Georgia get to decide how this all plays out.