The impact of Harris on the election result, however, is likely to be far more minimal. Vice presidents rarely move the dial significantly, so Harris' selection matters a lot more for 2021 and beyond than it does for 2020.
Many voters are likely to have no strong opinions of Harris, even at the end of the campaign. You can see this best in a study of past CBS News/New York Times polls as well as those from NBC News/Wall Street Journal, which ask voters their opinions of the candidates and provide explicit neutral and don't know categories.
Since 1980, an average of 35% of voters said they were unsure or neutral toward the vice presidential candidates. Among those who were not sitting vice presidents, that percentage climbed to 38%. In other words, there's a good chance that nearly two-fifths of voters won't even form opinions of Harris. That was the case for Vice President Mike Pence when he was running in October 2016.
In contrast, more than 80% of voters have formed opinions of the presidential nominees since 1980. And in the last 20 years, it's been closer to 90%, while for vice presidential candidates, it's been less than 65%.
The political science literature is also pretty clear about whether vice presidential candidates can win over certain groups for presidential nominees. The evidence is that they probably can't, which comports with the fact that many more voters don't hold opinions of the vice presidential nominees than they do of the presidential nominees.
Perhaps the way vice presidential nominees shake up the race the most is by how they reflect on the presidential nominees.
To that degree, Harris has compiled a liberal voting record in the Senate. Biden could be seen as more liberal by selecting her, though her record as the San Francisco district attorney and California attorney general makes her tougher to define than most. Biden's been doing well with liberals and moderates.
He has had trouble with some Black voters in the general election campaign. He's been running behind Hillary Clinton's margin in the final 2016 polls and has gotten into some hot water with recent remarks regarding "diversity" within the Black community. The selection of a Black woman younger than 60 may help to ameliorate some concerns that he doesn't understand the Black community. (It should be said, though, that Harris was never able to catch on with Black voters during the Democratic primary.)
Arguably, the biggest mistake Biden could have made was the selection of an inexperienced candidate. It could have made voters question his judgment. That's what political scientists Christopher J. Devine and Kyle C. Kopko believe happened to Sen. John McCain in 2008, after he picked Alaska Gov. Sarah Palin.
That charge against Harris, though, is unlikely to stick given her qualifications.
Indeed, the examples of Palin along with Dan Quayle in 1988 are good ones for understanding the scope of the vice presidential effect. Both were viewed unfavorably by voters, as they stumbled through questions about their readiness for the office.
