What is the political make up of the Supreme Court, and how are justices selected?

News that Supreme Court Justice Stephen Breyer plans to announce his retirement sent shockwaves through Washington and ended GOP hopes that the next court vacancy would occur while Republicans controlled the Senate.

The court underwent significant changes under the Trump administration, when over the course of four years, president Donald Trump oversaw the confirmation of three new justices to the bench, a third of the total bench, shifting the makeup of the court firmly to the right.

Mr Breyer’s retirement during the reign of a Democratic president means that the court’s overall make up will not change, for now. Thanks to the rules of the Senate, Mr Biden will be able to confirm his replacement in the months ahead, should he be able to unify the Democratic Senate caucus.

How do Supreme Court appointments work?

Judicial appointments to the Supreme Court are submitted to the Senate by the White House, where they begin their journey in front of the Senate Judiciary Committee. Nominees typically meet with most if not all senators on the panel privately before hearings begin, at which point the discussions about their record, competency, fitness and commitments to their values become public.

The US Constitution does not specify requirements for justices on the bench, but nominees typically have experience at the federal court level either as a justice, attorney, or both in the case of some such as recent addition Neil Gorsuch.

Hearings take place in the Judiciary Committee, which votes whether to send the nomination to the full Senate floor. Until 2017, 60 votes were required for a nomination to proceed through the Senate but that standard was eliminated in 2017 by then-Senate Majority Leader Mitch McConnell, who changed it to a simple 51-vote majority threshold.

Crucially, the Senate majority leader is the one who decides whether a nominee will receive a vote from the full Senate. Republicans used that rule to deny a vote to Merrick Garland, now US Attorney General, in 2020 when Justice Antonin Scalia’s death left a vacancy to fill during President Barack Obama’s final year in office.

Democrats currently hold an exact 51-vote majority in the Senate with the tie-breaking vote from Vice President Kamala Harris, meaning that Mr Biden’s nominee will be confirmed if the party is unified in their support for his pick.

What does the Supreme Court currently look like?

Conservative court watchers were thrilled by the Trump presidency, which saw appointments to the Supreme Court and federal judiciary at a staggering rate. To put the numbers in context, Mr Trump successfully oversaw the confirmation of three Supreme Court justices and 54 nominations to federal courts around the country, compared to two Supreme Court justices and 55 federal judges confirmed to the bench by former President Obama over eight years.

As a result the Supreme Court and the federal judiciary both shifted significantly to the right under Mr Trump’s term. Six justices make up what is commonly considered the conservative majority: Justices Samuel Alito, Clarence Thomas, Amy Coney Barrett, Neil Gorsuch and Brett Kavanaugh, as well as Chief Justice John Roberts. Three, Ms Barrett, Mr Gorsuch and Mr Kavanaugh, were confirmed under the Trump presidency.

On the court’s liberal side are Justices Breyer, Elena Kagan, and Sonia Sotomayor. The latter two were confirmed under the Obama administration, while Mr Breyer, 83, was nominated by Bill Clinton.

Chief Justice Roberts deserves special mention due to his role as a swing vote on some key cases; the most notable recent example was in 2012 when he sided with the liberal wing in a 5-4 decision to uphold a central provision in Mr Obama’s Affordable Care Act, better known as Obamacare, thereby saving the law from its most serious legal challenge.

Mr Roberts’ role has been diminished to some extent with the court’s further rightward shift. The “left vs right” model, however, is not a lock-step guide for understanding any of the court’s justices, and the political majorities of the Court are no guarantee of every case siding in favour of the dominant faction.

What’s next for the Supreme Court?

Two of the court’s justices – both conservatives, Mr Alito and Mr Thomas – are over the age of 70. That could potentially mean another vacancy during Mr Biden’s first term in office, which could be much more significant than the retirement of Mr Breyer in that it would give Mr Biden the opportunity to reverse the court’s rightward shift.

Any further confirmations to the court by Mr Biden would likely be contingent on Democrats retaining control of the Senate in November, already seen as an uphill battle for the party given the number of Democratic senators defending their seats this year. Senate Minority Leader Mitch McConnell has indicated that he would prohibit any nominations that occur “in the middle” of a presidential election to the bench should Republicans take control of the upper chamber.

Whether that means Republicans would block any Biden nominees, or just ones put forth in 2024 is unclear. But the Republican minority has also vowed retaliation for the now-failed effort by Democrats to reform the filibuster and make it harder for the minority party to block legislation.

"If my colleague tries to break the Senate to silence those millions of Americans, we will make their voices heard in this chamber in ways that are more inconvenient for the majority and this White House than what anybody has seen in living memory,” warned Mr McConnell earlier in January.

Mr Biden is expected to announce his nominee on Friday (25 February).