A new study estimates where President Obama’s pick, Judge Merrick B. Garland, and the candidates’ potential nominees, all federal appeals court judges, would fit on the ideological spectrum compared with current justices.

A new Democratic appointee — Judge Garland, or someone else named by Hillary Clinton should she win the presidency — would most likely vote much like the two more moderate members of the court’s four-member liberal wing, Justices Stephen G. Breyer and Elena Kagan, the study found. That would remove Justice Anthony M. Kennedy from his role as the median justice at the court’s ideological center.

“The possible Democratic appointees are bunched together, as are the current Democrats,” the study’s authors wrote. “Not much policy space separates them.”

That means Justice Breyer, along with Justice Kagan and the new justice, would find themselves near the court’s ideological middle. They would control the outcomes of closely divided cases.

The new study — prepared by Lee Epstein of Washington University in St. Louis, Andrew D. Martin of the University of Michigan and Kevin Quinn of the University of California, Berkeley — used a common and reliable political science measurement to make predictions about the potential nominees. This measurement is based on the ideologies of the presidents who appointed them to their courts and, when appropriate, of the home-state senators who supported their nominations.

Judge Garland has a reputation as a moderate. Under the study’s methodology, though, he is slightly more liberal than other potential Clinton appointees. “He is nearly as close to Ginsburg and Sotomayor as he is to Kagan and Breyer,” the study’s authors wrote, referring to the two most liberal members of the court, Justices Ruth Bader Ginsburg and Sonia Sotomayor.

A sampling of Donald J. Trump’s potential nominees tells a different story.

While the current Democratic appointees on the court are clustered together on the ideological spectrum, the Republican appointees are more widely dispersed. That is also true of Mr. Trump’s potential nominees.

Judge Thomas M. Hardiman, for instance, is only a little to the right of Justice Kennedy and so might share his place at the center of the court. Judges Raymond M. Kethledge and Diane S. Sykes, on the other hand, are to the right of Justice Samuel A. Alito Jr., who is quite conservative. But no potential nominee would challenge Justice Clarence Thomas, who is by some measures the most conservative justice in the modern history of the Supreme Court.

“Even with a new Republican appointee,” the study’s authors said, “Thomas is likely to remain somewhat isolated on the extreme right.”

A good way to predict how Supreme Court nominees will vote is to look at the political leanings of the presidents who nominate them. On the current court, there is a tight fit between the nominating president’s ideology and how justices have voted over the years.

President Bill Clinton’s ideology, as measured by his stances on legislation, predicted that Justice Ginsburg, whom he nominated in 1993, would vote in a conservative direction in 37.82 percent of cases. In fact, she has voted that way 37.12 percent of the time.

President Ronald Reagan’s ideology predicted that Justice Antonin Scalia, whom he nominated in 1986, would vote in a conservative direction 60.47 percent of the time. The actual number over Justice Scalia’s three decades of service was 61.23 percent.

None of the current justices’ voting records vary much from the predictions. The two outliers are Justices Kennedy and Thomas, and neither difference exceeds six percentage points. Justice Kennedy has been more liberal than Mr. Reagan, who nominated him in 1988, voting in a conservative direction 55.42 percent of the time. And Justice Thomas, at 64.12 percent, was to the right of President George Bush, who nominated him in 1991 and whose ideology predicted 58.29 percent conservative votes.