WASHINGTON — Chief Justice John G. Roberts Jr. has sat in the center seat on the Supreme Court bench since his arrival in 2005. But only this term did he assume true leadership of the court.

He made clear his influence in a pair of stunning decisions on Thursday, joining the court’s liberal wing in one and his fellow conservatives in the other. In providing the decisive votes and writing the majority opinions in cases on the census and partisan gerrymandering, he demonstrated that he has unquestionably become the court’s ideological fulcrum after the departure last year of Justice Anthony M. Kennedy.

The key parts of both decisions were decided by five-justice majorities, and the chief justice was the only member of the court in both.

The two rulings, one a rebuke to the Trump administration and the other a boon to Republicans, was consistent with Chief Justice Roberts’s insistence that politics should play no role in judging. “We don’t work as Democrats or Republicans,” he said in 2016.