One of the most important actions President-elect Donald Trump will take as president will be to nominate someone to fill the current vacancy on the U.S. Supreme Court. The court has functioned with only eight members since the death of Justice Antonin Scalia in February 2016.

Republicans in the U.S. Senate refused to consider President Obama's nominee for the position, arguing that the newly elected president should be able to fill that slot. Exit polls from the presidential election showed 21 percent of voters thought the Supreme Court nomination was the most important factor in their vote, and Trump got 56 percent of those voters. In order to please all the conservatives who voted for him, it is widely assumed that Trump will appoint a conservative to replace the very conservative Scalia.

While the initial Trump nomination will be extremely important, what is crucial is the possibility that Trump could name as many as four justices during his first term. In addition to filling the current vacancy, it is worth noting the advanced ages of several justices. Ruth Bader Ginsburg will be 84 in March, Anthony Kennedy is 80 and Stephen Breyer is 78.

Replacing Scalia with a conservative would not change much on the court, but replacing Ginsburg or Kennedy with a staunch conservative would alter the course of Supreme Court rulings for decades to come.

The current Supreme Court includes four liberal justices appointed by Democratic presidents, three conservative justices appointed by Republican presidents, and Reagan appointee Kennedy, whose swing vote usually sides with the conservatives but joins the liberals on some key issues such as gay rights and, most recently, affirmative action.

Since the Eisenhower presidency, the most liberal justices have been Republican appointees, including Chief Justice Earl Warren, Justice William Brenan, Justice Harry Blackmun and Justice John Paul Stevens. Today, we are in the historically odd situation of having all the liberals appointed by Democratic presidents and all the conservatives appointed by Republicans.

The U.S. Senate must confirm the presidential nominees to the court, and the confirmation calculus for Trump may get very complicated. Assuming that there will be more vacancies on the Supreme Court during his tenure, Trump may be considering which candidate he should nominate now, and which ones he should hold for potential future vacancies. The likelihood of confirmation may depend on the order of the nominations.

One key problem for Trump is the fact that the Republican margin in the Senate is so small. This amplifies the power of Republican moderates. The Republicans have 52 seats, and of course soon-to-be Vice President Mike Pence Michael (Mike) Richard PenceGOP short of votes on Trump's controversial Fed pick Pence seeks to boost Daines in critical Montana Senate race The Hill's Campaign Report: Trump's rally risk | Biden ramps up legal team | Biden hits Trump over climate policy MORE can vote in the case of a tie. But an extremist nominee might cause some Republican senators to oppose the nomination.

Take the abortion issue, for example. Sens. Susan Collins Susan Margaret CollinsThe Hill's Campaign Report: Biden asks if public can trust vaccine from Trump ahead of Election Day | Oklahoma health officials raised red flags before Trump rally Gideon leads Collins by 12 points in Maine Senate race: poll Senate leaders quash talk of rank-and-file COVID-19 deal MORE (R-Maine) and Lisa Murkowski Lisa Ann MurkowskiGOP ramps up attacks on Democrats over talk of nixing filibuster OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House Democrats tee up vote on climate-focused energy bill next week MORE (R-Alaska) are both pro-choice. Neither of them voted for Trump, and both have expressed concerns about House Republican plans to eliminate federal funding for Planned Parenthood.

If Trump were to nominate someone for the U.S. Supreme Court who held extreme anti-abortion views, would these two senators vote their policy preferences or would they follow party loyalty? Would their positions be different for the second or third potential Trump nominee?

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If Collins and Murkowski opposed a Trump appointee, then it would only take one more Republican senator to kill the nomination.

Democrats in the Senate have little incentive to support any Trump nominee, seeing how the Republicans were rewarded for their delaying tactics in 2016. Even Democratic senators from deep red states may not automatically support the Trump nominee, especially if the candidate is portrayed as an extremist who values ideology over legal analysis.

Democrats might try to filibuster the nomination, thus requiring 60 votes before the confirmation vote could be taken. Of course, the Republicans might then use the so-called

nuclear option and move to abolish the filibuster for Supreme Court nominees, like the Democrats did for lower court nominees several years ago. Democrats will certainly use the Supreme Court nomination vote in their 2018 midterm election campaigns.

Many Republicans still remember the failed nomination of Harriet Miers to the Supreme Court because she was perceived as not conservative enough, among other things. However, today, if a nominee is too conservative, that could cause problems among the few remaining moderate Republican senators.

Given that President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE could have more than one appointee, he must very carefully calculate a candidate's chances of confirmation before nominating them.

Mark C. Miller is a professor of political science at Clark University in Worcester, Massachusetts. He is also the director of the Law and Society Program there. During the 1999-2000 academic year, he served as a judicial fellow at the Supreme Court of the United States. In 2014-15, he held the Fulbright Distinguished Chair in American Studies at the University of Helsinki in Finland. He is the author of five books, including "Judicial Politics in the United States" (2015).

The views expressed by contributors are their own and not the views of The Hill.