It has become a familiar pattern. When the Supreme Court ruled last week that cities and states can ban nude dancing in clubs, the vote was close (6-3), and the conservatives won. And when the Supreme Court knocked down the White House’s hard-nosed efforts to regulate the tobacco industry last month, the justices were even more closely divided (5-4) – conservatives against liberals. In recent months, that 5-4 split has allowed the court’s conservatives to narrowly prevail in cases limiting the rights of defendants facing the death penalty, making it easier for police to stop people who flee when approached and restricting the federal government’s power to make states draw election districts that benefit black or Hispanic candidates.

That tenuous balance of power may soon change. The 5-4 split that has defined the court in recent years could be altered with the replacement of a single justice. And since it seems likely that one or more justices will retire in the coming four years, the next president may have the rare opportunity to sharply tip the court’s scales to the right or left, perhaps for decades to come. Until recently, the Supremes have remained a sleeper issue in the presidential race. But as the sparring intensifies, the battle over the future of the court could emerge as one of the most hotly contested issues of the campaign.