The left is starting to take aim at Democratic front-runner Joe Biden. At a conference this week, liberal activists repeatedly booed when told that Biden wanted to find "middle ground" on climate policy. When an audience member shouted "No middle ground!" Rep. Alexandria Ocasio-Cortez, D-N.Y., replied, "No middle ground is right!" and declared "I will be damned if the same politicians who refused to act come back today and say we need a middle-of-the-road approach to save our lives." Sen. Bernie Sanders, I-Vt., joined in her criticism, "There is no 'middle ground' when it comes to climate policy."

The left's issues with the former vice president go far beyond his position on climate policy. To the neo-socialists now driving the debate in the Democratic primary campaign, Biden's entire approach to politics -- reaching across the aisle and forging compromise built on consensus -- is anathema.

Biden's supposed heresy is that he believes in working with Republicans. He says on the stump that Trump is an "aberration" and predicts that if the president is defeated, Republicans will work toward bipartisan reform, which Biden insists is the only way to get anything worthwhile done. "This nation cannot function without generating consensus," he said in New Hampshire this week.

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Well, generating consensus is not what the left wants. It is not simply opposed to Trump. Many liberals believe, as Ocasio-Cortez has put it, that "capitalism is irredeemable." So for many Democrats, the Obama-Biden approach to governing is now considered too moderate. On climate, they don't want the government to simply invest in green energy like President Barack Obama did. They want to spend tens of trillions of dollars to replace every vehicle that uses a combustion engine, bring high-speed rail to every corner of the country, upgrade or replace every building in the United States and eliminate all fossil-fuel energy.

On health care, they no longer make a pretense of promising voters that they can keep their health plans like Obama did. They openly advocate abolishing private insurance altogether. Biden's support for a "public option" that would give Americans a choice of buying into a Medicare-like health plan is seen on the left as capitulation. There will be no choices in the brave new world of democratic socialism. We will have government-run health care for all, whether we want it or not.

Of course, Biden is no moderate. He is an old-fashioned, liberal Democrat. But to the Sanders and Ocasio-Cortez wing of the party, that makes him too far to the right -- and too willing to compromise with the far right. I saw Biden's willingness to do so up close when I worked on the staff of the Senate Foreign Relations Committee during the 1990s. As the ranking Democrat, Biden prided himself on his ability to compromise with committee chairman Jesse Helms, R-N.C., arguably the most uncompromising conservative in the Senate. Together, they passed legislation -- the so-called Helms-Biden Act -- to reform the United Nations and cut deals to restructure the State Department. "As chairman and ranking member, we passed some of the most significant legislation passed in the last 40 years," Biden explained during a 2015 speech. He continues to tout his relationship with Helms (who died more than a decade ago) on the stump as an example of how he can work with die-hard conservatives to get things done.

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Is this what Democratic primary voters want? Biden's lead in the national polls suggests it may be. But it is early. After all, at this time in 2015, Scott Walker, the Wisconsin governor at the time, appeared to be the front-runner for the Republican nomination and no one was taking Donald Trump seriously. Biden may be ahead for now, but all the energy inside the Democratic Party seems to be with the uncompromising left. It sees Biden standing in the way of its takeover of the Democratic Party. So as his lead in the polls expands, their efforts to stop him -- and his heretical calls for compromise -- will escalate.

"We have to unify this country," Biden said at a speech in Iowa earlier this month. "The other side is not my enemy, it's my opposition." How sad that has become a controversial statement.

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