The retirement of US Supreme Court justice Anthony Kennedy, announced Wednesday afternoon, is a seismic event of incredible significance. Up until this moment, the court has been reasonably balanced. It has four solidly liberal justices – Stephen Breyer, Sonia Sotomayor, Elena Kagan and Ruth Bader Ginsburg – and four solidly conservative justices – Clarence Thomas, Samuel Alito, John Roberts, and the Trump-appointed Neil Gorsuch.

But Kennedy, who was nominated by Ronald Reagan in 1988, has become known as the “swing vote” on the court. Sometimes he sides with the conservatives, including in Wednesday’s ruling which upheld the constitutionality of Trump’s travel ban, and in the controversial Citizens United decision. But he also often sided with the liberals on the court, especially on social issues like gay rights and abortion.

In 2013, he wrote the majority opinion for the court which ruled the Defence of Marriage Act (DOMA), which denied gay couples equal protection under the law, unconstitutional. In 2015, he wrote the majority opinion in a further ruling which effectively legalised gay marriage nationwide. He became known as a staunch defender of abortion rights, siding with the liberal justices in two key cases, 1992’s Planned Parenthood v. Casey – which could have overturned Roe v. Wade, the landmark 1973 decision which effectively legalised abortion in the United States – and 2016’s Whole Woman’s Health v. Hellerstedt, which overturned two Texas laws which the court found placed an “undue burden” on women seeking abortion.

Roe v. Wade remains a major bugbear for the American right. Opposition to it is the glue which ties evangelical Christianity to the Republican party, even under Trump. It is likely that their hope that Trump would nominate judges to America’s highest judicial body who would undermine Roe v. Wade is the core reason evangelicals held their noses and voted overwhelmingly for Trump in 2016 despite a cornucopia of scandals and the candidate’s clear lack of interest in religiosity (asked in August 2015 for his favourite bible verse, Trump couldn’t name a single one).

But during his campaign, Trump consistently promised to nominate judges who would overturn Roe v. Wade. And since Republicans currently control both houses of Congress, it is not only likely that the judge he will nominate will be much more of a doctrinaire conservative than Kennedy on abortion and other social issues, but also that whoever he chooses is likely to be confirmed by the Senate. That will mean that for the first time since the Roe v. Wade decision, the court will have a conservative majority that could overturn it. Conservative states where abortion provision has already been under pressure (seven states currently have only one remaining abortion provider) may now feel at liberty to ban abortion outright, safe in the knowledge that the Supreme Court will now likely uphold such a law.

During the campaign, Trump said that his victory would mean Roe v. Wade being overturned “will happen, automatically” if he won.

Democrats already consider Republicans to have stolen a supreme court seat. In 2016, Antonin Scalia, who was probably the court’s most conservative justice, died; then-president Obama nominated the impeccably-credentialled Merrick Garland to replace him, but the Republican Senate majority leader Mitch McConnell refused to bring the confirmation to a vote, citing the upcoming election as a reason to hold off. Garland’s confirmation was never heard; after Trump took office, he nominated Neil Gorsuch for Scalia’s seat.

Scalia was known as a “constitutional originalist”, a term which refers to an obsession with upholding the laws in the way that the original drafters of America’s founding legal documents would have wanted. In his tenure so far Gorsuch has made few friends on the court, because of his condescending and supercilious manner. In October 2017, he sarcastically suggested to his fellow justices that “Maybe we can just for a second talk about the arcane matter of the Constitution”, to which Ruth Bader Ginsberg snapped back, “Where did ‘one person, one vote’ come from?”

But really, Gorsuch is no more conservative than Scalia was in his outlook. Replacing Kennedy, on the other hand, because he was the swing vote around which the court turned, is a different beast entirely; the door is now open for Trump and the Republicans to drastically change the judicial landscape of America. Because a seat on the Supreme Court is a lifetime appointment with no term limits, Trump’s appointee could serve for decades – especially if he picks another young judge like Gorsuch, who, at 49, was the youngest ever nominee to the court.

Trump has been unpredictable in most other areas of policy-making. But on nominations to federal judicial positions he has largely outsourced his search to the Federalist Society, a right-wing think tank heavily funded by, among others, the Koch brothers (previous Republican presidents, including George W. Bush and Ronald Reagan, have also used the lists of potential judicial appointees that the Federalist Society produce). Based largely on Federalist Society recommendations, the White House published an ongoing list of potential Supreme Court judges back in November 2017, which is largely composed of conservative judges though it also includes Utah Republican senator Mike Lee, who was elected as part of the right wing Tea Party movement. The list is said to be of judges who pass Trump’s “litmust test” of wanting to overturn Roe v. Wade.

Of course, Trump being Trump, it is also possible that he might go off-script and nominate someone completely outrageous. In his book Fire and Fury: Inside the Trump White House, for example, Michael Wolff reported that Trump considered nominating former New York mayor and certifiable maniac Rudy Giuliani for Scalia’s court seat. Giuliani is now serving as Trump’s personal attorney, but it is by no means out of the question that Trump might nominate a political or personal ally, rather than a Federalist Society recommendation – who, though right-wing, are mostly at least experienced jurists – to America’s highest court.

Trump might be especially tempted to take this route if he is feeling under pressure from special counsel Robert Mueller’s investigation into Russian interference in the 2016 presidential election, which has already seen several former Trump team members, including former campaign CEO Paul Manafort and former National Security Adviser Michael Flynn, plead guilty to criminal charges.

After the Gorsuch outrage, and with some predicting a wave of Democrat victories in November’s mid-term elections, the fight over Kennedy’s court seat will likely become a political street-brawl of epic proportions, whether Trump picks a total outsider or one of the Federalist Society’s favourites.

Senate minority leader Chuck Schumer tweeted that Republicans “should follow the rule they set in 2016 not to consider a SCOTUS nominee in an election year. Sen McConnell would tell anyone who listened that the Senate had the right to advise & consent, & that was every bit as important as POTUS’ right to nominate.”

But Senate Democrats may not get a say in the matter. In 2017, the day before the vote to confirm Gorsuch, Mitch McConnell invoked the “nuclear option” which meant that Supreme Court nominations now need only a simple majority of 50 plus one, rather than a filibuster-proof 60 votes, to pass, echoing a move by Democrats in 2013 for cabinet and lower judicial nominations. The Republicans have 51 seats to the Democrats’ 49, though one Republican senator, John McCain, has been largely absent from Senate proceedings in this session after being diagnosed with cancer.

That means that Democrats need to convince only one Republican senator to vote against Trump’s nominee to sink their nomination. That might not be as difficult as you might think. Some Republican senators, including Arizona senator Jeff Flake, who is retiring at the end of his term, have been outspoken against the Trump administration; a couple are moderates who are pro-abortion rights, including Maine’s Susan Collins and Alaska’s Lisa Murkowski; and Nevada’s Dean Heller is facing reelection in a state which voted for Hillary Clinton in 2016. But this might be counteracted by Democratic senators running for reelection in conservative states; Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota, and Joe Donnelly of Indiana are all in this position, and the three voted with the Republicans in 2017 to confirm Gorsuch, as Vox points out.

All of this sets the scene for a vicious confirmation battle with sky-high stakes. Already, rhetoric is ramping up; Democratic senator Kirsten Gillibrand tweeted Wednesday night: “Any one of President Trump’s list of proposed SCOTUS justices would overturn Roe v. Wade and threaten our fundamental rights. I'll fight to make sure there are no hearings to replace Justice Kennedy until after the election.”

“This is our democracy,” she continues. “Let’s fight like it.”