Some candidates never learn. They go negative when their poll numbers are falling because their political strategists tell them that mudslinging will cut opponents down to size. Yet negative campaigners never seem to consider the possibility that they'll be the ones who wind up getting soiled, coming off as desperate, petty and unlikable.

Will someone please explain this to Hillary Clinton, whose campaign continues to embarrass itself with its childish attacks on Barack Obama?

Did I say childish? Some of this stuff is right out of kindergarten — where, thanks to the thoroughness of Clinton's opposition research, we now have the essential information that a 5-year-old Obama imagined himself running for president.

It gets worse. In recent days, Clinton and her campaign have made one tone-deaf blunder after another, and these mistakes seem to be paying dividends — for Obama.

Most polls show the Illinois senator leading Clinton in Iowa. And a recent CNN/WMUR poll showed him locked in a dead heat with her in New Hampshire.

Here's what the Clintonistas don't understand: A lot of Americans genuinely like Obama. Even if they don't support him, they admire him. And they're not going to like or admire Clinton if she or her surrogates attack him personally — especially in ways that are unfair and unoriginal.

The Clinton campaign is starting to sound a lot like the folks who, 15 years ago, went after a certain Arkansas governor.

First, Hillary claimed that Obama lacked foreign policy experience, something that George H.W. Bush said about Bill Clinton in the 1992 campaign. Then she questioned Obama's character, something that critics of Bill Clinton did all the time and not without cause. Hillary went on to challenge Obama's courage and convictions, which brought back memories of what Bill Clinton's liberal allies said about him when he pushed for reform of "welfare as we know it" and signed the Defense of Marriage Act.

Hillary Clinton defended her husband against those attacks. But now she's lobbing similar bricks at Obama.

Last week, Billy Shaheen, the co-chairman of the Clinton campaign in New Hampshire, crossed the line — and crossed into the surreal. Shaheen told a reporter that Obama's past could make him unelectable if he is the nominee and Republicans make an issue of the Illinois senator's youthful drug use.

This part of Obama's life isn't exactly a state secret. In fact, it was Obama who made it known in his autobiography that, as he struggled with identity issues as young man, "pot had helped, and booze; maybe a little blow when you could afford it."

That revelation hasn't hurt Obama up to now. But it could hurt anyone, Democrat or Republican, who tries to use it against him. Voters are likely to see it as a waste of time.

Besides, did Shaheen just sleep though the 1990s? Or does this Clinton adviser have any recollection of another candidate whose explanation of his drug use in the 1960s — and how he "didn't inhale" — became a punch line on late night talk shows? It's no wonder that Shaheen's comments landed with a thud, and that a Clinton spokesman quickly insisted they weren't "authorized or condoned by the campaign in any way." Shaheen himself apologized for the remarks and later resigned from the campaign.

But wait. As if bringing up someone's past drug use isn't low enough, Team Clinton went even lower. Speaking to the reporter, Shaheen helpfully suggested a series of questions that Republicans would likely raise about Obama's drug use: "When was the last time? Did you ever give drugs to anyone? Did you sell them to anyone?"

Stop the tape! Did you sell them to anyone? I don't recall anyone, no matter how much they hated Bill Clinton, ever suggesting that he was a drug dealer. But now an operative for Hillary Clinton is raising that question about Obama. Why do something like that?

I can't imagine that this has anything to do with the fact that Obama is black or that this is an ugly tactic intended to play on stereotypes about who uses drugs and who deals them.

Liberal Democrats would never do something like that. No matter how desperate they were.

Or maybe they would. Hey, this isn't kindergarten.

A lot of Americans genuinely like Obama. Even if they don't support him, they admire him. And they're not going to like or admire Clinton if she or her surrogates attack him personally — especially in ways that are unfair and unoriginal.

Ruben Navarrette Jr. is a columnist and editorial board member of The San Diego Union Tribune. Reach him at ruben.navarrette@uniontrib.com.

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