As it turns out, Democratic 2020 hopeful Cory Booker did not take the most solid strategy when he decided to attack Barack Obama's vice president as racist while himself winking and nodding at Louis Farrakhan.

For starters, John Lewis, the living civil rights legend and Democratic Georgia congressman, backed Biden against charges that the former vice president's civility towards segregationists indicated any sort of racism or complicity.

"I don't think the remarks were offensive," Lewis said after Booker mounted his short-lived smear campaign. If the nation's foremost authority on civil rights and black equality refuses to impugn Biden, who will and make the charges stick?

But even better for Biden are the polls, which prove that even if Biden's base is at least somewhat durable.

A Morning Consult poll found Biden's support unchanged from the previous week, at a solid (albeit by no means impenetrable) 38%. But his unyielding support from older black voters could prove devastating for the competition.

Black voters younger than 45 still prefer Biden by eight points over Bernie Sanders, second in the morning consult poll. But a whopping 59% of black Democratic primary voters older than 45 support Biden, bringing his overall support among black voters to nearly 50%.

In contrast, Booker and Kamala Harris, the two black candidates in the race, have just 5% and 9% support from black voters, respectively.

Democrats need to find a new line of attack, and fast. Although Democrats polled in a vacuum don't support septuagenarian candidates, that Biden and Bernie have maintained a combined near-majority consolidation of the field for the entirety of the race shows that the age attacks won't stick. Attacking Biden on policy has only served to encourage unforced errors, such as his Hyde Amendment flip-flop-flip, but even those only work if they affect Biden's behavior. If he stays resilient, it's unlikely that attacking Biden for opposing unpopular progressive fantasies would harm his decidedly liberal and centrist bases.

And now the race attacks have proven mostly a failure.

Decades of data demonstrate that a candidate cannot win a Democratic primary without support from black Americans. That Biden's excelling among two key backbones of the party, black voters and older voters who turn out in high numbers, spells danger for the rest of the field. If Biden's support among those two demographics, doesn't wane, the race is his to lose.