Ezra Klein argues that the 2012 presidential election is more important than most because it will decide the fate of the Affordable Care Act — and more broadly, the future of health care in the United States.

He’s certainly correct that the future of the ACA is at stake, although if Democrats wind up holding even in the Senate or even gaining a bit, then some sort of deal is still very possible. But as important as that is, I don’t think it’s the No. 1 thing at stake.

That thing is the Supreme Court.

It’s likely that the next president will replace at least one justice. If Mitt Romney wins next month and his party benefits from an improved economy by 2016 (not a certain scenario, but one that wouldn’t be surprising), then we’re talking about eight years and a very good chance of putting four justices on the bench.

In other words, not only is there a fair chance that the rough balance that the court has maintained for a couple of decades could be broken, but a new balance — either somewhat more liberal or quite a bit more conservative — could get locked in for a very long time.

Remember, there hasn’t been a strong ideological flip on the court since George H.W. Bush nominated Clarence Thomas to replace Thurgood Marshall. Some ideological movement isn’t unusual; Clinton nominee Ruth Bader Ginsburg is probably more liberal than the moderate Byron White, and solid conservative Samuel Alito replaced the moderate conservative Sandra Day O’Connor. Those were nudges, however.

If Romney wins the presidency and holds it for eight years, he very likely would replace not only moderate conservative Justice Anthony Kennedy (born in 1936) with someone closer to Alito, but he also would probably have the chance to replace either Ginsburg (both in 1933) or Stephen Breyer (1938). On the other hand, if Obama wins, it’s possible that he could wind up replacing at least one conservative justice, perhaps Kennedy or Antonin Scalia (also 1936).  

Granted, the reason that ideological change is rare is because most justices appear to time their resignations to assure a somewhat similar replacement. It’s far more likely that Scalia would retire with Romney in the White House than with Obama still there. But of course court vacancies are not always timed that way, and the court has been aging. There’s every possibility that we’re finally about to get some major ideological change.

The changes in the nation that could result from such a change on the court are breathtaking. And no, I’m talking not just about abortion or marriage, although it certainly would affect those. On every issue that’s at stake in the election, whether it’s the economy or executive power in national security or climate or yes, health care, a court in which Chief Justice John Roberts is the median voter would be enormously different from one in which, say, Elena Kagan is in the middle. Or, more to the point: A nation with a court in which Roberts decides split cases will rapidly become very different than one in which Kagan decides them.

None of this is guaranteed, neither the retirements nor the ideological bent of their replacements, but it is reasonably likely. I think it’s the biggest issue in the election, and I’m not sure it’s even close.