America is a divided nation. And we often seem desperate to find a leader who can unite us all in common purpose, who can help us put aside our petty squabbles and work together to create the future we all want. If only those partisans in Washington cared more about the country than about their party, then we could find our way out of the gridlock and back to the good old days when we were all on the same team... right?

Well guess what: There is no leader who can do that. So we ought to just stop asking.

In the vice-presidential debate this week, Tim Kaine and Mike Pence were asked what they planned to do to unify the country. I'm fairly certain we'll hear a similar question directed at Hillary Clinton and Donald Trump in one of their remaining two debates, if not in both. It's truly remarkable that by now we haven't realized that unifying us just isn't within the president's power.

But you have to give Clinton and Trump at least a little credit, because neither one of them has gone around claiming that by transcending partisan bickering, they'll be able to bring Democrats and Republicans together to solve problems. That's what Barack Obama promised to do, and what George W. Bush promised to do, and what Bill Clinton promised to do. They all tried, at least to some degree. And they all failed.

If you try to think of times when the country was truly unified, what probably pops into your head is World War II, when the Greatest Generation put its shoulder to the wheel to defeat the Axis. Or maybe the time right after the attacks of 9/11. George W. Bush, you might recall, had approval ratings over 90 percent for a brief time.

But Bush himself didn't bring us together. What brought us together was the moment of crisis and the external threat. Franklin Roosevelt was an extraordinary leader, but he didn't unify the country, either — lots of Americans hated him. Yet we persist in believing that a president, simply through the force of his will, can unify the country.

There's another myth that feeds this idea, one that says that the things that the parties disagree about aren't all that meaningful, and if they could just open their hearts they'd find common ground. We talk about "partisanship" as though it's just about preference, no more significant than whether you like chocolate or vanilla, or the Yankees or the Red Sox. But that's not true. There are times when people in politics face a choice between the good of the country on one hand and the good of their party on the other, but those are exceedingly rare. The vast majority of the time, Republicans sincerely believe that what's good for Republicans is what's good for the country, and Democrats feel the same way about their party.

When they disagree, it's often because they have incompatible values. Most Democrats believe it's inhumane for society not to guarantee a certain minimum standard of things like food, shelter, and health care; most Republicans don't. Most Democrats believe women ought to control their own reproductive lives; most Republicans don't. Most Democrats believe the wealthy ought to shoulder more of the burden of funding the things government does; most Republicans don't. These aren't just fleeting preferences — they spring from fundamental ideas about how the world works and how it ought to work.

And yes, we do live in a polarized age. But that's in large part because both parties have become more internally consistent about what they believe. There are problematic consequences for the government's ability to perform basic functions (even those that ought to be removed from ideology), but the differences are substantive and substantial.

So what's a president to do? The unfortunate answer is that all you can do is try to do as good a job as possible, even if it won't make for cross-party unity. Create a thriving economy and people's lives will improve — but don't expect the other side to give you credit. Bill Clinton presided over one of the greatest economic booms in American history, then his opponents impeached him for having an affair. Right now, perhaps in part due to an extraordinary run of job creation (about 15 million new jobs since the trough of the Great Recession), Barack Obama's approval ratings stand at just over 50 percent — which for a president in our time of disunion is about as good as it can get.

Both parties believe that the policies they advocate are both morally correct and practically efficacious; if you're right about that, then the things you do ought to produce good results. But don't expect the other side to admit that you were right and they were wrong all along.

What I'd like to hear is a presidential candidate answer the unity question by saying, "I wish I could unify the country, but I can't. All I can do is do the best job possible, even if I know that no matter what happens a healthy portion of the population is still going to hate me. But I'll try to do right by them too." That would at least be honest, even if the candidate would be pilloried for not buying into the myth of a unity that is always supposed to be awaiting us if only we can put aside our differences.