Supreme Court nominee Neil Gorsuch appears on Capitol Hill on Wednesday. (Zach Gibson/Agence France-Presse via Getty Images)

Donald Trump has nominated Neil Gorsuch, an originalist judge in the Scalia mold, to be the next Supreme Court justice.

The seat Gorsuch would occupy has been hotly contested for nearly a year, after the conservative Justice Antonin Scalia died unexpectedly last February. Senate Republicans refused to consider President Barack Obama's nominee, Merrick Garland, saying that the next president should make the pick. Gorsuch's confirmation would end the longest Supreme Court vacancy in nearly five decades.

Democrats and Republicans fiercely battled over the seat as the opening represented an opportunity to change the trajectory of the court in the coming decades. Had Scalia been replaced with a more liberal judge, the ideological balance of the court would have shifted to the left.

Instead, putting Gorsuch on the court would likely mean maintaining the Roberts-era status quo, according to Andrew Martin, a professor of political science at the University of Michigan. Martin and his colleague Kevin Quinn of the University of California at Berkeley have developed a widely used scoring system to measure the ideology of individual justices and of the Supreme Court itself, based on voting patterns.

“What Gorsuch represents is pretty much an even trade for Scalia,” Martin said via email. “Our analysis shows that he's located between Justices Alito and Thomas, just like Scalia. What this means is that Kennedy will remain the median, or pivotal, Justice if Gorsuch is confirmed.”

This notion of the median justice — the justice in the ideological middle, with four justices to the left of him or her and four to the right —  is a crucial one. Since you need that median justice to get a majority ruling in any given case, “the legal policy desired by the median Justice will (again, under certain conditions and voting procedures) be the choice of the Court's majority,” Martin and colleagues wrote in the North Carolina Law Review in 2005.

This makes the median justice a useful proxy for the court as a whole — in the Roberts era, you might say that as Kennedy goes, so goes the court. Martin and Quinn have calculated the ideological leaning of the court's median justice using the justices' votes every year going back to 1937. Here's what that looks like.

For much of the past 80 years, the court has leaned to the right ideologically. Key exceptions include the Warren court, which paved the way for the modern civil rights movement with its ruling in Brown v. Board of Education, and the Hughes court several decades earlier, which upheld the minimum wage and Social Security.

But after years of conservatism under Chief Justice Warren Burger and Chief Justice William Rehnquist, the court under Chief Justice John G. Roberts Jr. has taken a decidedly liberal turn in recent years. Part of that is due to big progressive victories, including the upholding of the Affordable Care Act in 2012 and the ruling against same-sex marriage bans in 2015.

But the court's rulings in any given term are also partly a function of the types of cases that lawyers bring to the court and the justices' decisions on which cases to hear. When the court is assumed to have conservative leanings (before Scalia's death, five out of nine of the Roberts court justices were appointed by Republicans), conservative litigants may be more likely to appeal cases to the court in hopes of attaining a favorable outcome.

That may mean that a conservative court's docket naturally tends to fill up with more conservative challenges to liberal policies (the Affordable Care Act, for instance). But if those challenges don't hold up in court, even a conservative Supreme Court may find itself handing down rulings that favor liberal policies.

If the leftward shift of the Roberts court holds up in coming years, it may have more to do with the cases the court decides to hear, rather than the influence of a Justice Gorsuch on his colleagues.

Far more interesting, Martin says, is what might happen to the court after the next justice departs. “If that [departure] is Kennedy or one of the four justices on the left, then the median would likely shift dramatically to the right.”

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