I'm hardly the only political observer who blames Hillary Clinton's general election defeat to Donald Trump in part on personal attacks on Clinton first made by Sen. Bernie Sanders, I-Vt., and his backers. Those attacks from her left laid the groundwork for copycat attacks lobbed by Donald Trump — and, in the process, helped hand the Supreme Court to the right-wing for a generation.

Don't believe me? Ask yourself who said what.

The answers: 1. Trump; 2. Sanders; 3. Sanders; 4. Trump; 5. Trump; and 6. Sanders.

And now, though the 2018 Democratic presidential primary has only just begun, those same long knives — mostly courtesy of supporters of Sanders' prospective candidacy — are out for outgoing Rep. Beto O'Rourke, D-Texas, charging that he is not a true progressive.

The reason for these pre-emptive attacks (which has the markings of a coordinated effort) in a spate of news and opinion articles in a variety of publications, is obvious enough: After losing the Texas Senate race to incumbent Ted Cruz, O'Rourke nonetheless has shot to the top in Democratic primary polls since Election Day, overshadowing both Sanders and another left-wing favorite, Sen. Elizabeth Warren, D-Mass.

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In politics you shoot up, not down.

Berniecrats seem determined to dust off the same destructive playbook this time around, even though the attacks against O'Rourke are flimsy and misleading. O'Rourke unseated a conservative Democrat in a primary and never tacked right in an election in deep red Texas. He has run in favor of federal legislation legalizing marijuana and the impeachment of President Trump, and although he fails their litmus test on free college tuition, their claims that his support is weak for Medicare-for-all don't match his record: As a Senate candidate, he said he would vote for it.

Bernie supporters have made questionable claims about contributions to O'Rourke from the oil and gas industry, as well as his support for certain Republican-sponsored House bills — but neglect to mention that the oil and gas money came mostly from low-level industry employees (hundreds of thousands of Texans are employed by the industry), and that O'Rourke broke ranks with his party less than the average Democrat.

The real problem for Sanders' supporters seems to be that this "Kennedyesque golden boy," as one has derided O'Rourke, seems perfectly poised to steal Sanders' thunder among millennials and white liberals with his fresh energy and personal charisma. Thus, it's not enough to disagree with O'Rourke; his persona and reputation must be dragged through the mud.

Democrats should greet this early maneuvering by Sanders' supporters with alarm. If Democrats cannot show such tactics — which will be used against any non-Sanders candidate, because no one can get to the left of a socialist — for what they are, they ignore them at their own peril.

Failing to end this internecine warfare will mean that all members of the Democratic Party running for its presidential nomination will face months of minuscule ideological litmus tests turned into character assassinations. The narrative, driven by the far left and lapped up by the press, will likely result in a nomination fight that could well devolve into the kind of pointless factionalism that will only help Republicans.

We've seen this movie before: Sanders' assault on Clinton's progressive credentials were pernicious in large part because they were not about policy disputes at all, but rather intended to falsely impugn Hillary's character and integrity.

The atmosphere online was even more toxic: Pro-Bernie message boards lit up with a montage of Hillary hate. Here, Hillary was a "corporate whore," a likely criminal in the email case and the cheating mastermind of a rigged primary.

No wonder "Lock her up!" later became so resonant.

In 2016, I ran a pro-Hillary SuperPAC which attempted to defend the candidate against false attacks, many of which came from or originated to her left. Though they were hardly in charge of our messaging, it was made very clear to us by our allies at her campaign headquarters that any efforts on our part to push back against the left-wing anti-Clinton brigades were unwelcome assistance; they feared alienating Sanders' voters.

That head-in-the-sand posture was ultimately self-defeating.

Today, Democrats are rightly laser-focused on picking a winner in 2020, and the stakes are just too high to let bad faith actors — whose real aim is to smear Democrats as no different than Republicans — stage inter-party schisms. If Sanders decides to run again this time, he should focus on policy and eschew character attacks on Democrats — and admonish his supporters to do the same. Otherwise, they put the core values we all share at risk, yet again.