(CNN) Former Vice President Joe Biden wanted to make a simple point in his interview with CNN's Chris Cuomo: Rep. Alexandria Ocasio-Cortez -- and others like her -- don't win competitive general elections.

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Which is, generally speaking, true! The Democrats who led the charge back to the House majority were, by and large, people running as moderates -- willing to take good ideas from any side of the political aisle to get stuff done for the public. Those, like New York's AOC, who represent overwhelmingly Democratic districts were free to advocate for large liberal projects (like the "Green New Deal") because they had no danger of any sort of Republican challenge in a general election.

Here's the hole in Biden's argument: He has to win a primary before he ever gets to a general election against President Donald Trump!

And if past is prologue, primaries are dominated by -- or at least controlled by -- the strongest partisans. Which, for Democrats, means liberals. (Who love AOC and are skeptical of Biden.

In the latest CNN poll of the 2020 field, Biden leads overall with 22%. But when you isolate Democratic voters by their ideology, you see that Biden is already struggling among liberal voters.

Among that group, he takes 12% -- which puts him behind California Sen. Kamala Harris (24%), Elizabeth Warren (20%) and Bernie Sanders (20%). Among those who identify as moderates or conservatives, Biden is way out in the lead with 31%, as compared to 11% for Harris 10% for Warren and 8% for Sanders.

Could Biden's strength among moderates and conservatives be enough to win him the nomination?

Maybe ... ? (said in the most skeptical voice possible). In the CNN poll, 50% of the Democratic and Democratic-leaning voters said they were moderates or conservatives while 48% called themselves liberals and 2% were unsure of their ideological leanings.

In theory, then, Biden could win all of the moderate/conservative voters, and even if the liberal vote lined up behind one other candidate (with Harris and Warren as the most likely possibilities), the former VP could still eke out the nomination. But, man oh man, is that a risky gamble.

Here's why: What pure ideological identification doesn't capture is enthusiasm and intensity. And that's where, usually, the most partisan members of a party far outstrip their more moderate brethren. Voting in the Iowa caucuses is an ordeal -- an extended process of sitting and debating with friends and neighbors, usually in either freezing cold or snow or both. (Iowa in February = not pleasant.) Ditto New Hampshire's primary. (Average daily temperature on February 1: 33 degrees .)

In those sorts of conditions, the candidate(s) with the most committed following tend to be the ones who prosper. And the candidates with the most devoted followings -- at least in Democratic politics -- tend to be the most liberal.

But, Hillary Clinton! -- you will say. She was clearly the more moderate choice of the two main candidates in 2016 (Sanders was the other) and she won. True! But the 2016 Democratic primary was far from a level playing field, with Clinton holding clear edges in everything from money to organization to, yes, the tacit support of the Democratic National Committee. Those structural advantages insulated Clinton from Sanders' strength among liberals.

Biden enjoys no such clear advantages. Yes, he is the frontrunner, but his lead is, at best, low single-digits. (Clinton led Sanders, at this point in the race, by 40 points or more.) And yes, Biden is the preferred candidate of non-white voters, but, again, his lead there is FAR smaller than Clinton enjoyed throughout the race.

Which all means that Biden's capacity to run a general election campaign in the primary -- as Clinton by and large did -- is significantly compromised. To win the nomination, Biden needs to fund ways to appeal to liberal voters, not alienate them. His strategy there appears to be regular reminders to voters that he was Barack Obama's vice president for 8 years. "I was vetted by him and selected by him," Biden said of Obama over the weekend in South Carolina. "I will take his judgment of my record, my character and my ability to handle the job over anyone else's."

That might work. Obama is a very popular figure among liberals. But it remains a very open question as to whether Obama's popularity will transfer itself to Biden.

The simple fact here is that while Biden may not need Ocasio-Cortez's endorsement to win the primary, he can't afford to become the candidate that AOC and those like her love to hate. Biden's position in the primary simply isn't strong enough that he can ignore the traditional calculus of needing to tack to the left to secure his party's nod before moving back toward the middle in the general election.

If he doesn't realize that reality, it could be a long next six months for the former vice president.