(CNN) It's hard for anything to come truly out of left field in a political world in which Donald Trump is the President of the United States.

And yet, when Hillary Clinton attacked Hawaii Rep. Tulsi Gabbard late last week (although not by name), it shocked me.

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"I'm not making any predictions, but I think they've got their eye on somebody who is currently in the Democratic primary and are grooming her to be the third-party candidate. She's the favorite of the Russians."

Clinton offered no proof to Plouffe for what is a very explosive claim aside from noting that Russian news agencies have occasionally praised Gabbard's longshot campaign for the Democratic presidential nomination . In the intervening controversy caused by her comments, Clinton hasn't put forth even any real explanation for why she decided to say what she said about Gabbard much less any proof of her claims.

And Gabbard's rivals for the nomination have, generally speaking, rallied around her. "Tulsi Gabbard has put her life on the line to defend this country," tweeted Vermont Sen. Bernie Sanders on Monday night. "People can disagree on issues, but it is outrageous for anyone to suggest that Tulsi is a foreign asset." On CNN Sunday, South Bend, Indiana, Mayor Pete Buttigieg said this of Clinton's attack on Gabbard : "I don't know what the basis is for that. But I consider [Gabbard] to be a competitor. I respect her service."

So, what gives? Why would Clinton -- one of the most powerful and recognizable leaders in the Democratic Party -- make a not-at-all-veiled allegation that Gabbard is being "groomed" by the Russians to run as a third party candidate and, in theory, make it possible for Trump to win a second term?

The short answer is: I don't know. Mostly because Clinton isn't saying.

Said Gabbard at the last Democratic debate, earlier this month in Ohio

"As president, I will end these regime change wars by doing two things: ending the draconian sanctions that are really a modern-day siege, the likes of which we are seeing Saudi Arabia wage against Yemen that have caused tens of thousands Syrian civilians to die and to starve, and make sure we stop supporting terrorists like Al Qaeda in Syria, who have been the ground force in this ongoing regime change war."

"I firmly believe that Tulsi Gabbard stands on that stage and is the antithesis to what the other 11 individuals stand for. Especially when it comes to issues such as foreign policy. There is no question that Tulsi Gabbard, of all the 12, is a puppet for the Russian government."

His evidence? "Her admiring somebody who we know to be a war criminal propping them up. I think that those are questions that will have to be answered."

There's no question that Russia is supportive of the Syrian government (and Assad). As The New York Times wrote Tuesday

"His jets patrol Syrian skies. His military is expanding operations at the main naval base in Syria. He is forging closer ties to Turkey. He and his Syrian allies are moving into territory being vacated by the United States.

And on Tuesday, President Vladimir V. Putin of Russia played host to President Recep Tayyip Erdogan of Turkey, for talks on how they and other regional players will divide control of Syria, a land devastated by eight years of civil war."

To play devil's advocate here: The fact that Gabbard is more sympathetic to Assad than the rest of the of the Democratic field is, without question, worth noting. But it is not definitive proof -- or anything like it -- that Gabbard is, in Clinton's words, a "favorite of the Russians" who the foreign government is grooming to split the Democratic vote in the general election and reelect Trump.

If Clinton has actual evidence that Gabbard is in fact being groomed by the Russians, she should by all means come forward with it. Because that would be a big story!

But if Clinton is simply making an allegation based on hearsay and rumor, well, that's a deeply irresponsible thing for someone in her position to do.