Much of the analysis that has been written about the 2020 Democratic primaries focuses on Joe Biden's vulnerabilities to attack from the left. But knocking Biden off of his perch as the front-runner may, counterintuitively, actually require his rivals attacking him from the right.

To be more a bit more precise, I'm not talking about adopting conservative positions ideologically, but employing lines of attack that you're more likely to see from President Trump in the general election than the usual ones you'd expect in a Democratic primary.

To give a recent historical example, back in the 2012 Republican primaries, there was a time when Newt Gingrich was written off for dead. He had come in fourth place in Iowa, and in fifth place in New Hampshire. Suddenly, he started lashing out at front-runner Mitt Romney's leadership of Bain Capital with rhetoric that sounded anti-capitalist and even Sanders-esque. He followed up with attacks on Romney for not releasing his taxes and described his plan for an optional 15% flat tax as allowing everybody to pay the "Mitt Romney rate."

At the time, many in the conservative media bristled at these lines of attack, which made him seem like a bitter sore loser. I, too, was initially puzzled — until it struck me that there was a method to Gingrich's madness. Up until that point, most of Romney's rivals had focused on attacking him for being non-authentically conservative. And sure enough, exit polls in Iowa and New Hampshire showed that those voters who were most interested in electing a true conservative, preferred other candidates. However, the exits also showed that a higher number of voters were seeking a candidate who could beat Obama, and Romney won those voters overwhelmingly.

So Gingrich deployed his new lines of attack ahead of South Carolina, where he also benefited from a debate confrontation with Juan Williams, and enjoyed a blowout victory in the state. And exits showed he won a majority of voters who saw the ability to beat Obama as their most important candidate quality. This was a short-lived success story, as Gingrich ultimately succumbed to his own many vulnerabilities as a candidate. But he had gone from dead to surging back into contention, even briefly overtaking Romney nationally.

Romney himself, it should be noted, had responded to the threat posed by Rick Perry the previous fall by attacking Perry's critical statements about Social Security.

In both cases, what these attacks had in common was that they broke the mold of trying to attack an opponent as insufficiently pure ideologically, and latched on to criticisms that one would expect the opposing party to make. At the same time, the lines of attack wouldn't undermine the attacker's chances within the primary. That is, there was a certain populist appeal to Gingrich's attacks on Bain, despite whatever philosophical defenses conservative or libertarian intellectuals may make for the virtues of venture capitalism. Social Security is broadly popular and the Republican primary electorate skews older, so as much as Romney's attacks rankled entitlement reformists, they didn't hurt him with the electorate — they just showed how Perry might be attacked by Obama.

If other Democrats are going to take down Biden, my guess is that at some point, they will have to pursue a similar strategy. An Ipsos poll released this week found that a 28% plurality of Democrats said the most important candidate quality was somebody who "can beat President Trump in the general election," compared to just 3% who said the most important quality was that the candidate was a "strong progressive."

That Biden isn't an ideologically pure "progressive" is not news to anybody. So making his deviations from the left the focal point of attacks against him has limited upside. To unseat him, his rivals are going to have to create reservations about his electability. The perception that he's the safest choice to take on Trump is what's fueling his polling leads. That needs to be shattered. And the only way to do that is attacking him as Trump would.

[Read more: Democratic rivals attack Joe Biden over report he prefers 'middle ground']