WASHINGTON — In Chief Justice John G. Roberts Jr.’s 11 years on the Supreme Court, his unfolding legacy has been marked by a debate over whether his very occasional liberal votes in major cases were the acts of a statesman devoted to his institution, a traitor to his principles or the legal umpire he said he aspired to be at his confirmation hearings.

This election could settle that debate.

If Donald J. Trump becomes president and follows through on his vow to appoint a conservative to replace Justice Antonin Scalia, Chief Justice Roberts will continue to lead a court dominated by five conservative justices. But the absence of Justice Scalia, the court’s longest serving and in some ways most dominant member when he died in February, means Chief Justice Roberts could lead in a more assertive way.

Were a liberal to replace Justice Scalia — whether it was President Obama’s pick, Judge Merrick B. Garland, or someone named by Hillary Clinton should she win the presidency — a majority of the justices would be Democratic appointees for the first time in almost 50 years. That would open a new chapter at the court, and leave Chief Justice Roberts, a Republican appointee with a generally conservative voting record, in the minority in many closely divided cases. And it could force him to choose between becoming a marginal figure or concluding that a new era on his court requires a new kind of leadership — and a move to the left.

“It’s been a long time since there was a chief justice who was in dissent across a wide range of important issues,” said Pamela S. Karlan, a law professor at Stanford.