Wedged between the Republican and Democratic national conventions next July will fall an event of greater long-term significance for the future of the republic: Supreme Court JusticeAnthony Kennedy’s 80th birthday.

Barring unforeseen events, Kennedy will become the third sitting octogenarian on the court — Justice Ruth Bader Ginsburg is 82, and Justice Antonin Scalia turns 80 in March. That will mark the first time since George H.W. Bush entered the White House more than a quarter century ago that a president has inherited three justices that old. At 78 by then, Justice Stephen Breyer will be close behind.

Of all the reasons why the high court will loom large in the presidential election — from a docket brimming with hot-button issues to intensive lobbying campaigns on the right and left — none looms as large as simple arithmetic and actuarial tables. The average age for Supreme Court retirements is just shy of 79. Since 1900, the average age of those who died while still serving was 69.

Next president could recast court

With the court narrowly divided, whoever wins the White House next November might enjoy the best chance to recast the high court in his or her image since Franklin Roosevelt did from 1937-43, when he named eight new justices. A Democrat such asHillary Clinton might get to replace Kennedy or Scalia, both named to the court byPresident Ronald Reagan. A Republican such as Jeb Bush might get to replace Ginsburg or Breyer, President Bill Clinton’s nominees.

In recent years and during the current presidential primary campaigns, Republicans and conservatives act as if they have the most at stake, while Democrats and liberals appear more sanguine — even though the court remains largely conservative. There are several reasons:

  • Republican presidents have made 12 of the last 16 high court nominations but have yet to win a reliable majority, a record that motivates conservatives. For more than 50 years, Democratic nominees have behaved on the bench as they were expected to behave, causing less consternation on the left.
  • After seven years of the Obama administration — and 15 of 23 years with a Democrat in the White House — Republicans have a laundry list of grievances. They want to block or reverse President Obama's health care, environmental and immigration initiatives. They want the high court to overturn rulings by federal appeals courts now dominated by Democrats' nominees.
  • Because she already is 82, there is a perception that Ginsburg will be the next to leave the court. Her refusal to do so while Obama is president gives conservatives hope that they can increase their majority on the court under a Republican president.
  • No seat is as important as Kennedy's, who sits to Chief Justice John Roberts' left, both literally and figuratively. Conservatives rue the day Reagan nominated him in 1988 after two preferred candidates ran into roadblocks, and they want to ensure a Republican president replaces him. Control of the court will hang in the balance.

“Kennedy was a compromise vote that has haunted the Republican Party for 30 years,” says Josh Blackman, a constitutional law professor at South Texas College of Law.

Ideologically unchanged since 1991

Not since 1991 has a Supreme Court seat changed hands ideologically, when Justice Clarence Thomas replaced Thurgood Marshall. Since then, only conservative JusticeSamuel Alito’s replacement of the more moderate Sandra Day O’Connor in 2006 affected the balance on the court. The last time a Democratic president was handed such an opportunity was in 1962.

These days, it wouldn’t be easy for a president to replace a liberal with a conservative, or vice versa. While Senate Democrats in 2013 changed the chamber’s rules so that judicial nominees could not be blocked by minority-party filibusters, they didn’t include the Supreme Court. That means 41 senators can block a nomination — though even Thomas, confirmed by a bare 52-48 margin, wasn’t blocked.

Many experts foresee three scenarios for the next Supreme Court nomination battle: Either a Senate controlled by the president’s party confirms the nominee by doing away with rules allowing the minority to filibuster, or a Senate ruled by the opposition forces the president to name a moderate or blocks any appointment until the next election. That last scenario could leave the court with only eight members for a protracted period.

“It’s very likely that that seat just stays vacant until there’s unified control” of the White House and Senate, says Ian Millhiser, senior fellow at the liberal Center for American Progress Action Fund. Blackman, a conservative, agrees. “We should be prepared to consider leaving a seat vacant,” he says.

A liberal court for decades to come

The stakes with four aging justices are huge: If a Democrat wins the White House and outlasts Kennedy or Scalia, the result could be a liberal court for decades to come, with an impact on issues such as abortion, campaign financing, political redistricting and voting rights.

Those stakes “are higher than most people appreciate,” says Eric Segall, a professor of law at Georgia State University and liberal blogger. When the ideological balance on the high court shifts, as it did when Thomas replaced Marshall, he says, “everything is different.”

Similarly, conservative watchdog groups have been seeking to prod GOP candidates on the issue, going so far as to run television ads in Iowa and New Hampshire denouncing even Chief Justice John Roberts as a turncoat for his votes upholding Obama’s health care law.

“Republicans realize from hard experience that you need to know more than just how to mouth the right incantations over your judges,” says Carrie Severino, chief counsel at the Judicial Crisis Network, which sponsored the ads.

Where the candidates stand

Many Republican candidates have answered the call. Sen. Ted Cruz has suggested justices face retention elections to keep their jobs, as some lower court judges do. Former Arkansas governor Mike Huckabee wants term limits. Jeb Bush has gone so far as to question his brother George W. Bush’s choice of Roberts, a fairly reliable conservative. Sen. Marco Rubio has called for “more Scalias and less Sotomayors.”

Democrats have reason to be satisfied with their recent nominees, so their wrath has been aimed more at the rulings of the Roberts Court. Hillary Clinton recently called itsCitizens United v. Federal Election Commission decision in 2010 upholding unlimited political spending by corporations “a grave error,” adding, “I will do everything I can to appoint Supreme Court justices who will protect the right to vote and not the right of billionaires to buy elections.”

If she wins the White House, she’ll likely have that chance. Even if no one retires, actuarial tables alone are catching up with the justices. To outlast a two-term presidency, Ginsburg and Breyer would have to reach 91 and 86; Scalia and Kennedy, 88.

While retired justice John Paul Stevens stayed on the court until he turned 90 and remains vigorous at 95, former chief justice William Rehnquist was the oldest in more than a century to die in office — at 80.