Democrats gain little by joining the GOP in peddling weird election theories.

Here's another test for the Democratic Party: How firmly can it reject the irresponsible accusation by former Democratic National Committee Chairman Howard Dean that Donald Trump was sniffling during the debate because he was using cocaine?

Dean made his claim last Monday night, and then stuck with it on Tuesday.

More is at stake here than one might think.

The temptation to embrace or tolerate irresponsible attacks is a constant for all political parties. And, contrary to what many liberals believe these days, there's nothing about conservatives that makes them uniquely susceptible to conspiracy-mongering.

Go back a decade or so, and you'll find plenty of liberals and leftists who trafficked in conspiracy theories about voting machines, just to cite one example.

It's especially tempting to indulge in such behavior when partisans see the other side doing it and, it seems, being rewarded for doing so. Dean may feel that the despicable rumor-mongering about Hillary Clinton's health that the Republicans and the Trump campaign have participated in makes it necessary to fight fire with fire.

Yet whatever the short-term rewards, the long-term consequences to both party and nation are dangerous.

Just look at the Republican Party.

For decades, its leaders have worked to train rank-and-file voters to think the media lies to them. And Republican-aligned talk shows and other conservative media have blared out that mainstream conservative Republican politicians are betraying the cause.

This drumbeat has promoted any number of wild fabrications. In the last few years, we've had the "birther" lie, the false claim that Barack Obama can't speak without a teleprompter, blarney about Benghazi — the list goes on.

Not all Republican leaders participated in promulgating these myths, but plenty did, and very few in the party had the spine to challenge them.

This decades-long effort to train Republican voters into believing things that weren't true wasn't the only reason the party wound up with Trump, a candidate without proper qualifications for the presidency or devotion to conservative ideas. But it was one reason, and maybe even a necessary condition. And Trump isn't the only consequence. This strategy nurtured the dysfunction in the Republican Congress as well.

The Democratic Party has done a much better job of pushing back against such temptations. Its leaders basically tossed former Rep. Cynthia McKinney out of the party for peddling conspiracies. The anything-goes brawler Rep. Alan Grayson of Orlando was just defeated for nomination to a U.S. Senate seat. Now they need to act again.

I'm not talking about hard-hitting attacks on opponents or even excessive spin, which is normal and can even be healthy. Campaigns should inform voters about the differences between the parties and their candidates. But inventing slurs is not normal or healthy. It can lead to poor governing, which means the whole nation suffers.

I don't believe Hillary Clinton has an obligation to denounce Howard Dean. But as the leader of her party, she would be wise to do so.

Jonathan Bernstein is a columnist for Bloomberg News.