Editor's note: Jeffrey Toobin is a senior legal analyst for CNN and a staff writer at The New Yorker magazine, where he covers legal affairs. His new book, "The Oath: The Obama White House and the Supreme Court," has just been published.
(CNN) -- What's the most important issue that neither President Obama nor Mitt Romney ever mention on the campaign trail?
The future of the Supreme Court.
Need proof? Try imagining how this election season would have unfolded if the justices had decided Citizens United a different way in 2010. Or suppose that the court had overturned, rather than upheld, the Affordable Care Act earlier this year. The Obama presidency, to say nothing of his campaign, might look very different today. As always, it was the Supreme Court that had the final word on what endures in the American government and in American life.
Citizens United transformed how campaigns can be funded, and the ACA case assured that 30 million people will soon obtain health insurance. But the stakes will be nearly as high in the other cases that will soon come before the justices.
The court begins its new session Monday, and this fall will consider the future of affirmative action in college admissions, in a case out of the University of Texas. And the justices are likely to decide whether the Voting Rights Act of 1965 is still constitutional. Same-sex marriage will probably come before them as well, either in the form of a challenge to the constitutionality of the Defense of Marriage Act or in the test of California's Proposition 8.
But these, of course, are only the cases that we know are in the Supreme Court pipeline. It's always hard to predict what the big cases will be. Who, for example, predicted that the court would decide the 2000 presidential election? We do know that the cases will be huge. Every important political and legal question in the country ultimately winds up before the justices.
Presidents pick justices; we just don't know when. The current Supreme Court is now a very old group. There are four justices in their 70s. Ruth Bader Ginsburg is 79; Antonin Scalia and Anthony Kennedy are 76; Stephen Breyer is 74. Justices also care deeply about who chooses their successors. Ginsburg has said that she would like to serve until she is 82, like her idol Louis Brandeis. Even so, we can be sure that she will leave some time next term, but only if her fellow Democrat, Barack Obama, wins a second term. By the same token, it's unlikely that Scalia or Kennedy, both Republicans, would leave during an Obama presidency.
But that's only if they have the choice. The melancholy fact is that people in their 70s don't always get to choose when they will retire. Nature sometimes intervenes. And predictions about retirements (like predictions about cases) are perilous. Jimmy Carter is the only president to serve a full term and not have the opportunity to name a single justice. Richard Nixon was president for only five and a half years (he had to leave early) but he had four appointments to the Supreme Court.
The opportunity to nominate Supreme Court justices is one of the most important ways for presidents to extend their legacies. John Paul Stevens retired in 2010 after 35 years on the court (he was appointed by Gerald Ford). Presidents can only serve for eight years, but justices now routinely serve for 30. And unlike presidents, justices always have the last word. As Justice Robert Jackson observed long ago, "we are not final because we are infallible, but we are infallible only because we are final."
With a little more than a month to go, it's not too late to ask the candidates to take a stand on their plans for the court. The president has already had two appointments, and he named Sonia Sotomayor and Elena Kagan. But what does Obama, a former law professor, think about the court? Does he believe in a "living" Constitution, whose meaning evolves over time? Or does he believe, like Justices Scalia and Thomas, that the meaning of the document was fixed when it was ratified, in the 18th century.
By the same token, what kind of justices would Romney appoint? Who are his judicial role models? Romney has praised Chief Justice John Roberts, but is the candidate still a fan even after the chief voted to uphold the ACA?
No one is asking these questions. But there are few more important things to know about our current and future presidents.
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The opinions expressed in this commentary are solely those of Jeffrey Toobin.