WASHINGTON — The latest urgent issue on the 2016 campaign trail is whether a Democrat or a Republican president will get to make as many as four Supreme Court appointments.

With four justices over the age of 77, the next president could dramatically reshape the court — upending its delicate 5-4 balance, and transforming the nation’s legal landscape. Candidates are stumping on it now, and hot-button issue advocates are both hopeful and fearful of what it could mean.

“We could lose the Supreme Court, and then there’d be a whole new litigation strategy coming from those who oppose marriage equality,” Hillary Clinton told a crowd of gay rights activists in Washington on Saturday.

“These picks are so important for the future of our country that if the next president gets to pick a Supreme Court justice they should pick someone — I would pick someone — with a proven judicial philosophy based on rulings and fight,” Republican hopeful Jeb Bush told The Greenville (S.C.) News on Friday.

The former Florida governor vowed to select more conservative justices than his father, former President George H.W. Bush. Bush’s selection of Justice David Souter, who largely aligned with the court’s liberal bloc, has been a source of Republican derision.

Though the issues may not be resonating with voters just yet, the impact could be ground-shaking.

“Every election, people say, ‘This is about the future of the Supreme Court and the Constitution.’ This time it’s true,” said Georgetown University constitutional law professor David Cole.

“If a Republican gets to replace … the two oldest Democrats, that changes the court to a solidly, solidly 5-4 conservative court,” Cole said. “If a Democrat gets elected and gets to replace … the two oldest on the Republican side, that also dramatically changes the court and now you have a solid 5-4 Democratic majority.”

The court is now almost evenly split by ideology, with four conservative-leaning justices, four on the liberal side and one — Justice Anthony Kennedy, known as the “swing voter” — in the middle and just as likely on almost any issue to go either way. Kennedy has cast the deciding vote in a host of key controversial decisions, from the ruling extending same-sex marriage rights nationwide to cases upholding abortion restrictions.

On Inauguration Day 2017, four of the current justices will be between the ages of 78 and 84: Justices Ruth Bader Ginsburg, now 82, Antonin Scalia and Kennedy, now both 79, and Stephen Breyer, 77. That means they’ll be 82 to 88 when the next president’s first term ends.

President Obama could get another opportunity to make an appointment should a vacancy arise in the next 14 months, but no justice has indicated any plan to retire. Despite calls from some liberal activists and newspaper editorial writers for Ginsburg to step down soon and let Obama replace her, she has insisted she’s staying put.

That means the next president could exert more power over the court than any other president in recent history.

“Given how old they all are, it’s not that unlikely, actuarially, that the next president will make those appointments — particularly if the next president serves two terms, then it’s virtually certain,” Cole said. “So this is a critical time.”

Supreme Court appointments have failed to emerge as a major issue among voters in past elections. In the upcoming general election season it could serve as a crucial motivator for the nominees of both parties, said Democratic political strategist Peter Fenn.

“It will further excite the base of each party,” Fenn said. “Conservative Republicans can get really worked up about Supreme Court appointments. And the liberal base of the Democratic Party can get really worked up.”

Renee Landers, Suffolk University law professor and board member of Planned Parenthood League of Massachusetts, said the replacement of just one justice — the court’s oldest — could affect the court’s abortion rulings dramatically.

“Ruth Bader Ginsburg is probably the most solidly reliable vote on the court when it comes to protecting reproductive rights,” Landers said.

Landers noted that even if a Democrat wins the next presidential election, ongoing Republican control of the Senate could be a barrier to liberal appointments, while a Republican could easily push through a conservative.

“I think if a Democrat makes the appointments, Citizens United would go,” said Bradley A. Smith of the Center for Competitive Politics, which favors lifting campaign contribution limits and fears a chilling effect on political speech if the 2010 Citizens United v. Federal Election Commission ruling is reversed.