In 1956 Adlai Stevenson, running against Dwight Eisenhower, tried to make the political style of his opponent's vice president, Richard Nixon, an issue. The nation, he warned, was in danger of becoming "a land of slander and scare; the land of sly innuendo, the poison pen, the anonymous phone call and hustling, pushing, shoving; the land of smash and grab and anything to win. This is Nixonland."

The quote comes from "Nixonland," a soon-to-be-published political history of the years from 1964 to 1972 written by Rick Perlstein. As Mr. Perlstein shows, Mr. Stevenson warned in vain: During those years America did become the land of slander and scare, of the politics of hatred.

And it still is. In fact, these days even the Democratic Party seems to be turning into Nixonland.

The bitterness of the fight for the Democratic nomination is bizarre. Both candidates still standing are smart and appealing. Both have progressive agendas. Both have broad support among the party's grass roots and Democratic voters. Supporters of each candidate should have no trouble rallying behind the other if he or she gets the nod.

Why, then, is there so much venom out there?

Most of the venom I see is coming from supporters of Mr. Obama, who want their hero or nobody. I'm not the first to point out that the Obama campaign seems dangerously close to becoming a cult of personality. We've already had that from the Bush administration. We really don't want to go there again.

What's particularly saddening is the way many Obama supporters seem happy with the application of "Clinton rules" -- the term a number of observers use for the way pundits and some news organizations treat any action or statement by the Clintons, no matter how innocuous, as proof of evil intent.

The prime example in the 1990s was the press coverage of Whitewater. A small, failed land deal became the basis of a multiyear, multimillion-dollar investigation which never found any evidence of wrongdoing yet it became a symbol of the Clinton administration's alleged corruption.

During the current campaign, Hillary Clinton's entirely reasonable remark that it took LBJ's political courage and skills to bring Martin Luther King Jr.'s dream to fruition was cast as some kind of outrageous denigration of King.

And the latest prominent example came when David Shuster of MSNBC, after pointing out that Chelsea Clinton was working for her mother's campaign -- as adult children of presidential aspirants often do -- asked, "doesn't it seem like Chelsea's sort of being pimped out in some weird sort of way?" Mr. Shuster has been suspended, but as the Clinton campaign rightly points out, his remark was part of a broader pattern at the network.

It's a pattern that goes well beyond the Clintons. For example, Al Gore was subjected to Clinton rules during the 2000 campaign: Anything he said, and some things he didn't say (no, he never claimed to have invented the Internet), was held up as proof of his alleged character flaws.

For now, Clinton rules are working in Mr. Obama's favor. But his supporters should not take comfort in that fact.

For one thing, Hillary Clinton may yet be the nominee -- and if Obama supporters care about anything beyond hero worship, they should want to see her win in November.

For another, if history is any guide, if Mr. Obama wins the nomination, he will quickly find himself being subjected to Clinton rules. Democrats always do.

But most of all, progressives should realize that Nixonland is not the country we want to be. Racism, misogyny and character assassination are all ways of distracting voters from the issues, and people who care about the issues have a shared interest in making the politics of hatred unacceptable.

One of the most hopeful moments of this presidential campaign came last month, when a number of Jewish leaders signed a letter condemning the smear campaign claiming that Mr. Obama was a secret Muslim. It's a good guess that some of those leaders would prefer that Mr. Obama not become president; nonetheless, they understood that there are principles that matter more than short-term political advantage.

I'd like to see more moments like that, perhaps starting with strong assurances from both Democratic candidates that they respect their opponents and would support them in the general election.