What a lovely spectacle the Democratic candidates put on!

OK, so it's not so lovely. With little in the way of policy differences, Barack Obama and Hillary Clinton have begun spiraling downward into the muck of personal attacks and vicious smears. For those pessimistic Democrats who wondered for a year how their party could snatch defeat from the jaws of victory, the answer now seems clear: Adopt a primary system in which nobody can win, and keep your top two candidates snarling at each other instead of Republican John McCain.

CYNTHIA TUCKER MY OPINION • E-mail Tucker Recent columns: Obama's next stop: Main St.  Gov. Palin, welcome to the club  No reason to fear detainees  Promote racial diversity fairly  Those who live in glass ...  Market won't fix Rx care  Follow the money in Rx debate

In a rare violation of implicit rules for intra-party feuds, Clinton has suggested that McCain would make a better commander in chief than Obama, and her husband has managed to imply that Obama isn't patriotic. (And you thought only Republicans did that to Democrats!) Meanwhile, an Obama supporter has compared Clinton to Joseph McCarthy, while another managed to invoke the infamous blue dress.

As the Democrats kneecap each other, their respective supporters have grown increasingly bitter, with higher and higher percentages of each camp declaring they could not vote for the rival if he/she became nominee.

Because the Democrats are concerned about egalitarianism — unlike the GOP — they devised a complicated system of proportionality for awarding their primary delegates. If their formula were the same as that used by the GOP, they'd award delegates in a winner-take-all fashion, just as in a general election campaign, when the candidate who wins a state gets all the Electoral College votes. Under that system, Clinton would be on her way to the nomination, having won key big states, including California, Texas and New York.

But that's not the system the Democrats devised. And no matter how much Clinton partisans talk up the importance of big states, the party decided years ago not to lean heavily on large states to determine its nominee.

It's too late to change the rules the Democrats agreed to. While party leaders might want to revise their rule book before the next presidential election, that's not something they should attempt now. Every candidate knew before the campaign season began that they'd be using complicated mathematics devised to increase diversity while also preserving electability. (The two notions may be diametrically opposed, but that's Democrats for you.)

At this point, that mathematical formula has virtually ruled out a Clinton victory through the primary system. Superdelegates have the power to make her the nominee, but they seem increasingly unlikely to do so.

On Saturday, New Mexico Gov. Bill Richardson joined the dozens of superdelegates who have decamped to Obama — many since he lost Texas and Ohio. And last week, one of Clinton's own aides, speaking without attribution, told Politico.com reporters Jim Vandehei and Mike Allen that she has no more than a 10 percent chance of winning.

If the Democratic Party wants to keep alive its chances of winning back the White House, Howard Dean and other party bigwigs need to approach Clinton about folding her campaign. Now.

There is much to recommend McCain. While his voting record is deeply conservative, his opposition to torture and his commitment to shut down Guantanamo are testimony to his principles. During a heated primary season, he refused to pander to conservative voters on immigration. He has shown a willingness to reach across the aisle.

Nevertheless, he is either flat wrong or hopelessly out of touch on two of the top five issues — the war in Iraq and the economic downturn. He has admitted that he doesn't know much about the economy. As for the war, he has said he doesn't care if U.S. troops remain in Iraq for a hundred years. At the moment, he draws strong support in the polls for his ability to handle the war, but that's likely because most voters don't know about his plan for a Hundred Years War.

With so much riding on the next election, Americans deserve a choice between two well-matched candidates with very different proposals on foreign and domestic issues.

But if superdelegates don't choose a Democratic nominee soon, voters will get an unblemished Republican and a badly tarnished Democrat.

• Cynthia Tucker is the editorial page editor. Her column appears Wednesdays and Sundays.