Almost four years ago, I was watching Sarah Palin rile up a Clearwater, Fla., crowd with anti-Obama broadsides when a spectator let loose a bloodcurdling cry of "kill him!"

To his credit, John McCain realized the Obama hatred was getting out of hand, and a few days later, when a woman at one of his events called Barack Obama an "Arab," McCain did one of the most honorable things in his political career. "No, ma'am," he said, taking the microphone from the woman and enduring some boos from supporters. "He's a decent, family-man citizen that I just happen to have disagreements with on fundamental issues."

Now that the year is again divisible by four, the anti-Obama hatred is flaring anew. But I worry that Obama's current opponent doesn't have the strength of character to push back against the most dangerous voices on his side.

The latest sign of trouble came Monday, when a woman speaking at a Mitt Romney event in Euclid, Ohio, said that Obama was operating outside of the Constitution and "should be tried for treason." Many in the crowd of 500 applauded this call for the commander in chief of the United States to be charged with a capital offense.

But Romney didn't push back against this outrage. Instead, he said he thinks the Constitution is "brilliant" and mentioned nothing about treason. Only when reporters pressed him later did Romney state that he did not, in fact, think Obama should be put on trial for being a traitor to his country.

This was just the latest instance of Romney being unwilling to confront the darker forces of the right:

Days earlier, Romney made scant effort to defend one of his aides, Richard Grenell, who had been hired only weeks before to serve as a foreign policy spokesman. Conservative groups complained noisily because Grenell is openly gay. Romney declined to push back publicly against the conservatives, and Grenell resigned.

This followed Romney's unfortunate response two months ago to Rush Limbaugh's claim that a Georgetown University student who testified about birth control was a "slut" and a "prostitute." The candidate declined to rebuke Limbaugh, saying only that "it's not the language I would have used."

Go back further to the Republican debates, when Romney, like the other presidential candidates, didn't make any real-time attempt to distance himself from ugly behavior by the debate audiences: lustily applauding the record number of executions in Texas, cheering when a moderator spoke about a hypothetical 30-year-old dying because he lacked health insurance, booing when a gay service member asked a question about the "don't ask, don't tell" policy.

Some take Romney's reticence to challenge the right as evidence that he is the "severe conservative" he claimed to be. I suspect it has more to do with weakness: He has been so abused by the right for so long that he lacks the confidence to offend conservatives. Either way, the result is the same, and Romney remains without the Sister Souljah moment Bill Clinton achieved with his 1992 criticism of a black rapper's racial provocation.

Romney has company in this weakness. Rick Santorum declined to correct a woman who said at one of his campaign events that Obama is an "avowed Muslim." But Santorum was a protest candidate; Romney, barring catastrophe, will be the Republican nominee. Romney's aides correctly point out that he isn't responsible for everything his supporters say, but Romney himself invited this standard when he called on one of his primary rivals, Rick Perry, to disavow an introductory speaker who had called Mormonism a "cult."

And Romney does push back against audience members when he wishes to. When a woman challenged him from the left at a campaign event last August, he cut her off, saying, "You had your turn, madam! Let me have mine." But when it comes to intolerance on his side, he is not so brave. Last year, Romney, to his credit, dismissed the Obama birth-certificate issue, saying, "The citizenship test has been passed."

But earlier this year, Romney appeared with Donald Trump to receive the endorsement of the most famous man associated with the "birther" movement. Trump, who still hasn't entirely abandoned the birth-certificate issue, campaigned for Romney, taping phone messages and doing radio interviews.

Now Romney has another chance at a Sister Souljah moment. This weekend, he's giving the commencement address at Liberty University, the Christian school founded by Jerry Falwell. Think he'll use that forum to tell conservatives to stop questioning Obama's religion and patriotism?

Don't count on it.

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