Young activists demonstrate outside the Supreme Court on June 28 as President Trump prepares to choose a replacement for Justice Anthony M. Kennedy, who is retiring. (J. Scott Applewhite/AP)

Justice Anthony M. Kennedy’s announced retirement has led many to speculate about how Trump’s second appointment to the Supreme Court will affect national politics. Most think the court will lurch right. But is that really certain?

This is what the court looks like ideologically right now

Political scientists identify the justices’ ideological positions through statistical analyses of their votes in Supreme Court cases. In the figure below, the justices’ ideological views are estimated based on votes cast through the 2012-2013 term. I scored Justice Neil M. Gorsuch based on his close agreement with Justices Clarence Thomas and Samuel A. Alito Jr. on close cases. This isn’t the only way to place justices along the left-right spectrum, but political scientists generally agree that the justices line up roughly as follows:


Current Supreme Court justices on an ideological left-right scale.

As you can see, Justices Ruth Bader Ginsburg, Sonia Sotomayor, Stephen G. Breyer and Elena Kagan sit on the left. On the right are Gorsuch, Alito and Thomas. Chief Justice John G. Roberts Jr. is one step to the left of this conservative cluster. He occasionally has sided with the court’s liberals, as he did when he wrote the opinion in 2012 upholding the constitutionality of the Affordable Care Act. He also wrote for a moderate majority just this month in Carpenter v. United States, in which the court decided that police need warrants for searches of cellphone records.

Most striking is Kennedy, standing alone between the liberal and conservative camps. That position allowed him to form a winning majority coalition with either the conservative or the liberal bloc of justices. Consistent with how political scientists explain court decision-making, Kennedy’s status as the “median” justice gave him power on the court. In cases decided by a vote of 5 to 4, Kennedy for years sided with the winning coalition vastly more often than any of the other justices.

Can Trump move the court decisively to the right?

Trump would like to move the court to the right. That gives him strong incentives to nominate an extremely conservative justice. But keep in mind the importance of the median justice. As long as Trump selects a nominee who is more conservative than Roberts, Roberts will become the new court median.

Most observers assume that that would make the court reach far more conservative decisions. But even though justices are very predictable ideologically, they are not perfectly predictable.

Sometimes court cases involve legal or other principles so important to a jurist that he or she defects from typical coalition mates. There are plenty of examples. Roberts voted with the liberals to uphold the Affordable Care Act. Kagan voted to strike down the ACA’s mandated expansion of Medicaid to all 50 states. And so on.

To reduce the chances of this, conservatives seeking a more conservative court will push Trump to appoint a very conservative nominee whose views are significantly to the right of Roberts’s. Conservatives want to avoid a replay of Kennedy, who frequently sided with the liberals on the important cases — despite being fairly conservative.

Will the Senate go along?

Trump can only put an extreme conservative on the bench with the support of a majority of the Senate. Right now the GOP has a 51-49 majority in the chamber. Sen. John McCain (R-Ariz.) is away for cancer treatment, and not expected to return. That means that even with Vice President Pence there to break a tie, the GOP has only 51 votes. If one Republican senator defects, and if no Democrats cross the aisle to vote for the nominee, Trump’s pick would not be approved.

In the figure below, I have added the senator who is currently the chamber’s ideological median, Susan Collins (R-Maine). Moderate senators such as Collins have enormous power when their party holds a slim majority, as we saw when McCain, Collins and Sen. Lisa Murkowski (R-Alaska) from their party to vote down ACA repeal. Therefore it wasn’t a surprise when, this weekend, Collins announced her requirements for supporting a Supreme Court nominee.


Current Supreme Court justices (and Sen. Susan Collins on an ideological left-right scale.)

To put Collins on the same scale as the justices, I rely on a statistical model that incorporates positions that she and all other members of Congress took on particular Supreme Court cases, such as amicus briefs submitted for particular cases before the court, statements in congressional debate about Supreme Court cases and any congressional votes directly relevant to cases. I can then produce ideological estimates for members of Congress that are on the same scale as those for justices.

Based only on ideology, Collins should theoretically vote against any nominee who is more conservative than Kennedy and Roberts.

Nonetheless, Collins may still vote for an extreme Trump nominee, out of the fear that the Democrats could win control of the Senate this November and halt any Trump nominee. Or Collins may decide that she might not get reelected if she alienates her conservative base.

But would adding an extreme conservative to the court always result in far-right decisions?

Justices often operate differently than elected officials. The court has only nine people and is often closely divided. That means that often a single justice can carry the day when voting contrary to his or her usual ideological predisposition. And of course, justices don’t face reelection, which gives them more latitude to shift from their usual positions

So although we talk about justices in terms of their ideological views, we should keep in mind that they have incentives and a track record of bucking ideological and partisan expectations.

Michael A. Bailey is the interim dean of the McCourt School of Public Policy and the Colonel William J. Walsh professor of American government at Georgetown University. With Forrest Maltzman, he co-wrote “The Constrained Court: Law, Politics and the Decisions Justices Make” (Princeton University Press, 2011).