Young African-American voters are picking Bernie Sanders over Hillary Clinton in the Democratic primary race, revealing a generational rift in which millennials are eschewing a penchant among their elders for the Clintons dating back to the 1990s, when Bill Clinton ran away with the black vote.

They are showing up in too few numbers to give the Vermont senator much of a boost. Nevertheless, that many millennials prefer Sanders' progressive agenda shows there are cracks in Clinton's domination of the African-American electorate.

Fifty-two percent of black voters between the ages of 17 and 29 voted for Sanders, compared to Clinton's 47 percent, according to a new NBC News analysis of exit polls in 25 primary states. However, black Americans under 30 represented only 3 percent of the vote in these states.

Clinton holds a sizable lead over Sanders among African-Americans in every other age group: among voters 30-44 she leads by a margin of 70-29 percent, ages 45-59 a margin of 85-14 percent, and voters who are 60 and above by a margin of 89 to 9 percent. Of the total electorate, these groups represented 6 percent, 9 percent and 7 percent respectively.

Older voters typically vote in greater numbers than young voters. Case in point, the 2012 general election, where 41 percent of 18-24-year-old Americans of all races voted according the U.S. Census, and 57 percent for voters ages 25 to 44. By comparison nearly 68 percent of U.S. citizens 45-64 voted. Among African Americans, there was a smaller gap between young and old voters in 2012. Forty-nine percent of black voters between the ages of 18 and 24 voted; 65 percent among those 25-44; and 72 percent of 45-64 year olds went to the polls.

A key portion of Clinton's support base depends on the black vote, which over the course of the primary season has helped the former secretary of state maintain her "firewall" of support in the South where there is a large non-white electorate. This helped her achieve huge victories in states like South Carolina, where Clinton won by nearly 50 percent, thanks in part due to her receiving 87 percent of the black vote.

And while Sanders tends to perform best in states with majority-white populations like Vermont and Washington State, some recent polling reveals that Sanders can be competitive with other minority groups. Recent polling shows Clinton and Sanders neck-and-neck with Hispanic voters.