(FORTUNE Magazine) – Some say they're tweedledum and tweedledee (or Tweedledum and Tweedledumber, as it was put by a particularly acid writer on I forget which fringe of the political spectrum). Despite the divisiveness of the war issue, lots of voters--more than usual, it seems to me, from right and left--are complaining that Bush and Kerry are more alike than different, leaving us a choice with not much to choose from.

Let's get used to it. The trend toward increasingly similar candidates is a big one, not just a quirk of this election, and it won't be turning around anytime soon. The explanation is right before our eyes, though you won't find it amid the campaign coverage in the newspaper. Rather, it's in the supermarket.

To understand what's going on, don't think Bush vs. Kerry, think Coke vs. Pepsi. We Americans are the undisputed world champions of consumer marketing, and of course a presidential election is a particularly dramatic and large-scale exercise in that discipline. Pols have understood that reality for ages, at least since Ike hired BBD&O in 1956. Back then the marketers took the candidate the party handed them and did their best to sell him. But over time they've extended their influence further back into the selection process. The major-party nominees are no longer chosen through political horse-trading at the national conventions, but through primaries, where political marketing is the game. In addition, the political marketers have been advancing their craft for so long now that today's candidates have been immersed in it since they first ran for student council. The result is a generation of candidates shaped through personal development and the selection process by the principles of modern marketing.

And here is one of the key insights of that science: In category after category, the market leaders are virtually identical. "Virtually" is important. They aren't absolutely identical. They're just very, very close. Whether it's potato chips, toothpaste, disposable diapers, or any of hundreds of other products, the pattern holds. You can tell the difference between Coke and Pepsi if you care about soft drinks, but the difference is minuscule. That's because endless research has established what consumers like most, and straying too far from those specs is asking for failure. Yet thanks to skillful marketing, millions of cola drinkers hold violently emphatic preferences for Coke or Pepsi.

Similarly, millions of voters are ardently pro-Bush or pro-Kerry, though the differences between the candidates aren't nearly as dramatic as the differences in opinions about them. Before us stand a couple of Skull and Bones men from Yale, and while cartoonists love to caricature them, a visitor from Mars might think we had almost perfected cloning. On the great issue of taxes, both candidates want to make Bush's tax cuts permanent, with the exception that Kerry wants to return the top rate to its pre-cut level, a difference of less than four percentage points. On Iraq, Kerry says even now he would have authorized the use of force, though his policy would have been more "thoughtful and sensitive" than Bush's.

Those and the other differences between the nominees are the differences between Coke and Pepsi, the market leaders. Go outside the parameters, and you're just a curiosity. Howard Dean was obviously Jolt Cola. Ralph Nader is Dr Pepper, attracting a steady, single-digit market share that will never increase. Dennis Kucinich was organic carrot juice. And so on.

As our political marketers become more expert, the homogenization of the major-party nominees is intensifying. Everyone learned a lesson from the greatest presidential election blowout ever, Reagan's thrashing of Mondale in 1984, when Mondale carried only his home state of Minnesota. Every election since has been closer than the last (with a minor blip in the trend line when Clinton won by a slightly greater margin in 1996 than in 1992), leading to the 2000 race, the closest in U.S. history, in which the two major candidates achieved virtually identical market shares.

I don't pretend to know how this election will turn out or how close it will be. But I know the marketers are getting better every day. Their latest technique: running MRIs on voters' brains while showing them campaign commercials (it's called neuromarketing). I'm not sure where it will all lead, but I'm pretty sure where it won't lead--to more striking differences between major-party nominees.

GEOFFREY COLVIN, senior editor at large of FORTUNE, can be reached at gcolvin@fortunemail.com. Watch him on Wall $treet Week With FORTUNE, Friday evenings on PBS.