In recent months, some on the left have suggested that the next time Democrats have control of Congress and the White House, they pack the Supreme Court — that is, increasing the total number of seats on the court in order to allow a (hypothetical) Democratic president to immediately appoint one or more new justices. In case you're wondering: Yes, Congress can change the number of seats on the Supreme Court. It wouldn't even be all that difficult.
Court packing gained currency on the left, in large part, as a reaction to the Republican blockade of Merrick Garland, Barack Obama's final Supreme Court nominee. Obama nominated Garland to replace Antonin Scalia in 2016; however, Senate Majority Leader Mitch McConnell refused to put Garland up for a vote in the Senate, instead waited until Donald Trump won the presidency, which allowed him to nominate a conservative judge, Neil Gorsuch, instead.
McConnell ultimately blocked Garland's nomination for longer than any other Supreme Court nominee in U.S. history, and never even held hearings on his nomination. Given that Obama was given the American people's blessing in 2012 to appoint Supreme Court justices for the next four years, many people see the Garland blockade as a bad faith violation of basic democratic norms, and consider the seat stolen (Such folks might also point to the fact that leading Republicans acknowledged that Garland was qualified to sit on the court and would have been confirmed if he'd been given a vote).
As a result, some on the left — including, most notably, attorney and potential 2020 presidential candidate Michael Avenatti — have called for Democrats to campaign on court packing, and to actively pursue it if given the power to do so. Avenatti has demanded that all Democratic candidates for president in 2020 promise to pack the court if they're elected, and the idea has received even more attention in light of Brett Kavanaugh's controversial confirmation. But how would court packing actually work?
The Constitution established the Supreme Court, but says nothing about its size. As such, changing the number of seats on the court requires nothing more than passing a piece of legislation through both houses of Congress and having the president sign it. Although often referred to as "radical" proposal, court packing isn't a new idea, and it's been done before. Republicans packed the court during the Civil War in 1863, temporarily increasing the number of justices to 10, and Franklin Roosevelt attempted unsuccessfully to pack the court in 1937.
It's almost a given that if Democrats tried to do this in the current political climate, Republicans would attempt to filibuster, which would theoretically require Democrats to wrangle 60 votes in the Senate to pack the court. But that's easy to circumvent: If Senate Democrats were truly committed to packing the court, they could simply eliminate the legislative filibuster, which the majority party can do if it wishes for any given congressional session.
From a mathematical standpoint, Democrats only need simple majorities in the House and Senate in order to pack the court. Of course, it would be foolish for them to try and do this during a Republican presidency, as that would give Trump (or Mike Pence) the power to appoint the new justices. Practically, then, Democrats would need to control the White House as well as Congress.
Whether Congressional Democrats will actually pursue such a plan is unclear, which is why activists on the left are pressing them to do so. Having said that, the earliest opportunity Democrats would have to court pack would be 2021, so elected Democrats and their base have a long time to hash this issue out. But from a purely technical standpoint, it's completely doable — as long as Democrats win the next two elections.