They fail to deliver their home states. Don't bring people of their faith to the polls. And a woman hasn't boosted turnout among voters of her gender. So what good are vice presidential nominees?

Not much, say researchers and political scientists. But California Sen. Kamala Harris' elevation to be Joe Biden's running mate could have a bigger impact in both the short term for the election, the long term for people who look like her and the future of the Democratic Party.