One-issue voters have always seemed like cranks to me.

It’s natural enough to have priorities, but who thinks one of them is more important than all the other political issues combined?

This presidential election, though, is different.

A reasonable person could vote on the basis of future Supreme Court nominations alone — because on almost everything else that matters, the differences between the candidates are going to be vanishingly small when put into practice.

Start with some obvious truths that no one is much mentioning in the news media’s frenzied effort to generate excitement. Because the election is going to be close, whoever wins will have little or no mandate to act definitively.

The House and Senate probably will be split, meaning that either Barack Obama or Mitt Romney will have to govern in the center.

Partisans on either side will tell you that the policy differences are legion, which is true. It’s just that presidents don’t get to implement the majority of their most important domestic policies without Congress, and Congress is not going to back either side in making radical changes.

Both candidates want to lower the deficit, but neither controls the economic cycle, which will either generate millions more jobs and higher tax revenues, or won’t.

On foreign policy, as the lackluster final debate revealed, there is essentially no daylight between the candidates. It won’t be in either president’s interests for the U.S. to be dragged into a Middle Eastern war, or to take a soft line on China’s monetary policy.

But wait, you say, surely the two men have radically different personalities, and divergent approaches to government. Their personalities differ, of course, and Obama may trust government a shade more than Romney.

But let’s be honest: These are two highly pragmatic, highly intelligent, highly competitive, very tall men. Each has had a special aura around him at least since his student days. They both went to elite private high schools, fancy colleges and then graduate school at Harvard.

There is a reason that the word “pragmatist” has been attached to each of them for many years. Obama supporters accuse Romney of having no fixed beliefs, but of course pragmatism embraces that stance as open-mindedness. Romney backers say Obama is actually a leftist who thinks government can do a better job of building wealth than private industry.

Obama, however, is no leftist — just ask the leftists who are sorely disappointed by his centrist presidency.

When it comes to Supreme Court appointees, though, the differences really are going to be stark — and they will last for a generation. Somehow the campaigns have failed to remind us that four justices are 74 or older, meaning they will be at least 78 by the end of the term.

Justice Ruth Bader Ginsburg is already 79, with Justices Antonin Scalia and Anthony Kennedy not far behind at 76 and Justice Stephen Breyer at 74.

One hopes of course they all live long lives, but the notion that all four still will be willing and able to serve the next four years is preposterous. Several will retire and be replaced — and even one replacement could fundamentally change the configuration of the court.

If Romney becomes president, Ginsburg certainly will do all she can to remain in the saddle. But if she were to have to retire for health reasons (she has been treated for colon cancer and pancreatic cancer), the court would become ineluctably conservative.

The current 4-to-4 split with Kennedy as the swing vote would change into a stable 5-to-3 conservative majority, with Kennedy no longer important.

Under Romney, Scalia might retire to give a Republican president the chance to replace him with someone young and comparably conservative. That would consolidate the conservative majority of Chief Justice John Roberts, Justice Samuel Alito and Justice Clarence Thomas (now 64) for as long as Thomas stayed healthy.

Or consider the scenario where Obama is re-elected and either Scalia or Kennedy is replaced by a relatively more liberal justice. That would, in turn, create incentives for Ginsburg and Breyer to retire, which would allow the possibility of a five-justice liberal majority in which Justice Sonia Sotomayor, now 58, would be the eldest.

The Senate can constrain the president’s pick in either direction so that we do not get a radical justice on either side. But just because we will not have a Robert Bork does not mean we will get an Anthony Kennedy.

Roberts and Alito both got substantial Democratic support when they were confirmed. No president of either party can be denied a reasonably moderate Supreme Court nominee who is not an ideologue.

In today’s world of detailed vetting, it is increasingly unlikely that an apparent conservative could be nominated who would either become a liberal over time or come to be seen as one, as happened with David Souter.

And in Supreme Court history, liberal justices have come to be seen as conservative only when the court itself moved far to the left (which is what happened to Felix Frankfurter, who believed he had not moved at all).

The justices of the future will decide our nation’s course on abortion, affirmative action, civil liberties and gay rights, not to mention big-ticket policy items such as health care.

Our Founding Fathers never imagined such a role for the court, but for better or worse, it is here to stay.

Vote your conscience this time around, but for heaven’s sake, vote on the Supreme Court. It’s going to matter.

Noah Feldman, a law professor at Harvard University and the author of “Scorpions: The Battles and Triumphs of FDR’s Great Supreme Court Justices,” is a Bloomberg View columnist.

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