The resignation of Anthony Kennedy gives Donald Trump the opportunity to select his second Supreme Court justice. Who will he pick?

Until fairly recently, presidents often made idiosyncratic choices with respect to judicial ideology when selecting future justices. President Dwight D. Eisenhower, for example, chose William Brennan because he wanted both a Democrat and a Catholic to maintain partisan and geographic balance on the Court; Brennan famously went on to become a liberal stalwart on the Court. But as Supreme Court nominations have taken more prominence in American politics, such ideological “mistakes” have all but disappeared.

One way that parties and presidents can signal their intentions on future nominees is the use of policy litmus tests — that is, a promise to appoint justices who will decide cases on a given policy in the direction favored by the party. As part of our ongoing research on the politics of Supreme Court nominations, we recently read and coded every party platform and presidential acceptance speech since 1928. Here is what we found.

First, Republican Party platforms are now open and explicit about policy litmus tests for Supreme Court nominees. Here are some quotes from the most recent Republican Party platform (in 2016):

Only a Republican president will appoint judges who respect the rule of law expressed within the Constitution and Declaration of Independence, including the inalienable right to life and the laws of nature and nature’s God, as did the late Justice Antonin Scalia. … Only such appointments will enable courts to begin to reverse the long line of activist decisions — including Roe, Obergefell, and the Obamacare cases. … The confirmation to the Court of additional anti-gun justices would eviscerate the Second Amendment’s fundamental protections. … We support the appointment of judges who respect traditional family values and the sanctity of innocent human life.

Trump’s acceptance speech embraced an explicit gun test:

The replacement for our beloved Justice Scalia will be a person of similar views and judicial philosophies. Very important. This will be one of the most important issues decided by this election. My opponent wants to essentially abolish the 2nd amendment. I, on the other hand, received the early and strong endorsement of the National Rifle Association and will protect the right of all Americans to keep their families safe.

Second, policy litmus tests are a relatively recent phenomenon. Before 1968, policy litmus tests were unknown, and may have been seen as outside the bounds of normal American politics. In 1968, however, Nixon broke the norm by pledging that he would appoint only “law and order” judges.

But the dam really burst in 1980. In that year’s platform, and in every subsequent platform, the Republican Party imposed an abortion litmus test (in the anti-abortion direction) on judicial appointments to the Supreme Court. In a few platforms, the policy demands also embraced “law and order” (1988, 1996).

Then, in 2012, the Republican Party agenda added opposition to the Affordable Care Act as a policy requirement for nominees, and in 2016 support for gun ownership. This figure shows the timing and types of litmus tests from 1928 to 2016. It makes clear that Republicans (coded in red) have employed litmus tests earlier and more often than Democrats (coded in blue).

Third, there is a big difference between Republican and Democratic litmus tests for Supreme Court nominees. As shown in the figure, Democratic Party platforms and acceptance speeches rarely impose specific policy litmus tests outside abortion, and even there are somewhat inconsistent. More frequent are calls for diversity and respect for minority rights.

Thus, Republicans appear to be playing “constitutional hardball.” While Democrats have focused less on policy litmus tests, we found they have been more likely to emphasize the need for racial and gender diversity on the Supreme Court; this divergence is consistent with the broader asymmetry in ideological commitments across the two parties.

Why do policy litmus tests matter? Presidents have a strong incentive to deliver on them. The sudden appearance and proliferation of litmus tests in the party platforms reflects the prominence of interest groups and issue enthusiasts in the parties after the nominating reforms of the 1970s. (This proliferation, as we have shown, is coincident with a rise in the number of interest group active in Supreme Court confirmation politics).

As scholars of parties have pointed out, presidents cannot run competitive primary contests without the enthusiasm, shoe leather, and money offered by organized interests and issue enthusiasts. At the present juncture, a surefire way to alienate the groups a Republican nominee needs would be to ignore their heartfelt demands for Supreme Court nominees.

In light of the historical patterns, what can we predict about Trump’s forthcoming nominee? While there will be sententious statements about open minds and justices being umpires, the nominee will likely be anti-abortion, pro-law and order, pro-gun rights, anti-Affordable Care Act, anti-gay marriage, and generally conservative. This is what key coalitions in the Republican Party demand, and it is what President Trump (with some help from the Federalist Society) will likely deliver.

Charles Cameron is a professor of politics and public affairs at Princeton University. John Kastellec is an associate professor of politics at Princeton University.

Even in a “normal” election year, the political calculations surrounding the vacancy on the Supreme Court would be complicated. A president in his final year is facing a Senate opposition that is determined to prevent him from nominating a new justice who would substantially shift the ideological balance of the Court toward its liberal wing. But, as is now clear, this is far from a normal year — and the prospect of Donald Trump being the Republican nominee may significantly change the game.

When Senate Majority Leader Mitch McConnell announced within hours of Justice Scalia’s death that the vacancy “should not be filled until we have a new president,” his logic was clear. Rather than give President Obama a chance to fill the new seat, McConnell and his Republican caucus would take their chances on a Republican president taking office in January, allowing the new president to name a nominee and thereby maintain the Court’s ideological balance.

In the event that a traditional Republican candidate won the presidency, the type of resulting nominee was all but certain. A Republican presidential win would likely mean the Senate would stay in Republican hands, given the correlation between Senate and presidential elections. Which means the Republican president would appoint a reliably conservative justice, and the Republican-controlled president would confirm him or her (and would likely end the filibuster for Supreme Court justices, if the Democrats attempted to block a vote on the nominee, given the Democrats did the same thing with respect to lower federal judges in 2013).

How do we know this? Stung by so-called “stealth nominees” like Anthony Kennedy and David Souter, who voted more liberally over the course of their tenures than expected, the conservative legal movement has worked to ensure that Republican presidents appoint reliably conservative justices. President George W. Bush’s nomination of John Roberts and Samuel Alito illustrated this point — and Bush’s nomination of Harriet Miers turned out to be an exception that proved the rule, as her nomination died swiftly due to opposition from fellow conservatives who were worried about her conservative bona fides.

But would a President Trump stick to this script? During the campaign, Trump has sounded like a conservative with respect to the Supreme Court. He has stated that his favorite justice is Clarence Thomas, and last month he suggested two conservative judges on the Courts of Appeals — William Pryor and Diana Sykes — would be good candidates to be elevated to the Supreme Court. (In addition, Trump has echoed conservative criticism of Chief Justice John Roberts for his two decisions upholding Obamacare.)

But President Trump would have a free hand to nominate whomever he chooses — since he is only loosely tied to the Republican brand and party apparatuses, Republicans need to worry about his commitment to appointing a reliably conservative nominee. (Indeed, Ted Cruz ran a television ad highlighting Trump’s past support for abortion rights as argument that Trump cannot be trusted to appoint Supreme Court justices.)

Which brings us back to McConnell and the Senate Republicans. Let’s assume they face four potential scenarios:

• A President Trump who would appoint a reliable conservative justice, with the Senate controlled by Republicans
• A President Trump who would appoint a moderate justice, with the Senate controlled by Republicans. (We assume there’s no chance Trump would win the presidency and Republicans would lose control of the Senate.)
• President Hillary Clinton facing a Republican-controlled Senate
• President Hillary Clinton facing a Democratic-controlled Senate

What type of nominee would each scenario bring? We can only speculate, albeit in an informed manner based on political science research. This is what the current Court looks like, in terms of the justices’ ideology (based on their Martin-Quinn scores):

Let’s assume a reliable Trump would nominate someone like Justice Alito. What an unreliable nominee would look like is, of course, unknowable, but for our purposes let us assume he or she would look like Justice Kennedy. A President Clinton backed by a Democratic Senate would likely follow the example of Obama and nominate someone like Justices Kagan or Sotomayor. Finally, if Clinton faced a Republican Senate, the dynamic would be similar to the current one.

In an earlier blog post, based on our research with Jee-Kwang Park, we estimated that President Obama would have to appoint a moderate nominee well to the right of Justice Breyer to have a chance of winning 50 votes in a Republican-controlled Senate (see also this post by Tom Clark and John Patty). Let’s assume, then, that Clinton would nominate someone halfway between Breyer and Kennedy. Here’s what that looks like.

Notice that the “Clinton with Republican Senate” outcome is not much different from the “unreliable Trump” outcome.

Now we need to know the likelihood of each scenario. Current betting odds give Hillary Clinton about a 70 percent chance of winning the general election. We assume there’s a 50 percent chance the Democrats take the Senate, if Clinton wins. (Obviously shifting that number in either direction would change the calculus.)

The final piece of the puzzle is: What is the likelihood of Trump being reliable? Once we have all these pieces, we can weight the expected outcomes (that is, the ideology of the nominee) by the likelihood of each scenario coming to pass.

What the following graph shows is the expected nominee ideology, as the probability of Trump being reliable ranges from 0 (full rogue) to 1 (utterly reliable). This is given by the solid line, which, not surprisingly, shows that the expected nominee becomes more conservative as Trump becomes more reliable. The dashed line shows what a moderate Obama nominee might look like; again, this is the same as the “Clinton with Republican Senate” scenario. Once the probability of a reliable Trump drops below 30 percent (depicted by the dashed vertical line), a moderate Obama nominee would become more attractive to McConnell than taking the chance of Hillary Clinton getting to appoint a very liberal justice.

Obviously, these numbers are just guesses, and a lot may change. Furthermore, McConnell may stick to the pure obstruction strategy this year whatever happens, as he must weigh not only the type of nominee who replaces Scalia but also the electoral fate of any Republican senators who voted to confirm an Obama nominee. But the prospect of a Trump presidency should make him think twice.

Charles M. Cameron is a professor of politics at Princeton University. Jonathan Kastellec is an assistant professor of politics, also at Princeton.