WASHINGTON — The retirement of Justice Anthony M. Kennedy is likely to thrust Chief Justice John G. Roberts Jr. into the court’s ideological center, making him the deciding vote on abortion, gay rights and affirmative action cases alongside a newly solidified conservative majority.

For the past dozen years, Justice Kennedy has sat in the ideological middle of the polarized court, with four liberal justices to his left and four conservative ones to his right, according to scores based on their voting patterns. His retirement will almost certainly mean that position goes to Justice Roberts, potentially encouraging him to be more moderate.

The chief justice, a conservative nominated by Republican president George W. Bush, has drifted slightly to the left in recent years, drawing howls of protest from activists on the right who have complained that he has proved to be a disappointment. But other than two votes upholding the Affordable Care Act, Chief Justice Roberts, 63, has reliably sided with the court’s other conservatives.

With Justice Kennedy’s departure and the likelihood that President Trump will succeed in winning confirmation of a conservative successor, the question is whether Chief Justice Roberts — an incrementalist who is passionate about preserving the institutional integrity of the court — will inch further toward the center.