How Donald Trump’s rise complicates the Supreme Court nomination battle

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.

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