Former Vice President and presumptive Democratic nominee Joe Biden announced Sen. Kamala Harris as his running mate today. Here's a look at what this choice means and ... what it doesn't.

It's a historic choice, with the potential for even more history to be made.

Harris is the first Asian American and the first Black woman in American history to be a general election candidate for president or vice president for either of the two major political parties. (Harris's mother was born in India, her father in Jamaica. They met as graduate students at the University of California at Berkeley, in the 1960s.) Harris is just the second Black person (after Barack Obama) and the fourth woman (after Democrats Geraldine Ferraro in 1984 and Hillary Clinton in 2016, and Republican Sarah Palin in 2008) to be on a presidential ticket for one of the two major parties. If she and Biden win the November election, she would be the first Asian American, the first woman of any race or ethnicity and the second Black person in U.S. history to be vice president or president.

Harris's selection is the latest sign of the increasing diversity of the Democratic Party. Democrats last had an all-white, all-male ticket in 2004, with then Sens. John Kerry and John Edwards. This vice presidential process, with Biden committing to choosing a woman fairly early on and then choosing a Black woman, suggests the Democrats may rarely in the future have a ticket of two white men. They may also rarely in the future have a ticket of two white people (as in 2016 with Clinton and Tim Kaine) or two men (as in 2012, with Obama and Biden).

It's another illustration of the power of Black Americans in the Democratic Party.

A clear plurality of Black voters favored Biden throughout 2019, helping keep the former vice president near the top of polls through most of the Democratic primary race. Black voters in South Carolina and then other states, particularly in the South, strongly supported Biden and played a key role in his winning the nomination despite lackluster showings in Iowa, New Hampshire and Nevada.

After it was clear Biden would be the party's nominee, a lot of prominent Black Democrats - mostly notably Rep. James Clyburn, the highest-ranking Black Democrat in the House - pushed for Biden to pick a Black woman as his running mate. With Harris's selection, their wish was granted.

It's another defeat for the party's left wing.

With Sens. Bernie Sanders and Elizabeth Warren losing the nomination contest, many liberal activists pushed Biden to pick Warren as his running mate. They were unsuccessful. Harris has a fairly liberal voting record in the Senate, but she's not nearly as far to the left as Warren. Harris hasn't called for the breakup of Facebook, for example, or supported a wealth tax.

The Democratic Party is moving left ideologically. But liberal activists may have a hard time getting their main policy goals adopted, even if Democrats control the House, Senate and the presidency next year. The most important figures in Washington in 2021 might be Biden, Harris, House Speaker Nancy Pelosi and Senate Democratic Leader Charles Schumer, all of whom have kept some distance from the party's left wing.

Let's pause there for a moment. I could have made the preceding three points - the history of the selection, the power of Black voters and the loss for the Left - and even used nearly the exact same words in some of those sections if Biden had selected one of the other Black women who was reportedly being considered for vice president. That list includes former Georgia House Minority Leader Stacey Abrams, Rep. Karen Bass, Atlanta Mayor Keisha Lance Bottoms, Rep. Val Demings and former National Security Adviser Susan Rice.

The implications of Harris's selection, however, become more complicated when you look at Harris specifically, as opposed to the selection of a Black woman more generally. So here's what's not clear:

We don't know if Harris will help or hurt Biden win the general election.

Research suggests that the electoral impact of vice presidential candidates is fairly limited. I would expect Harris to follow the same pattern. Harris is a sitting senator who was competent during the debates she participated in during the 2020 Democratic primary process, so she is unlikely to make huge gaffes that raise questions about why Biden selected her (in the way that Palin became a problem for John McCain in 2008). At the same time, a senator from California doesn't provide an obvious electoral boost in a key swing state. Also, overall, Harris wasn't a particularly effective candidate when she ran for president last year.

So the most likely outcome is that Harris's selection doesn't change much - either Biden keeps his current lead and wins the presidency or Trump comes back based on factors that don't have a lot to do with Harris. But politics is dynamic, so I'm not sure that prediction will come true.

We don't know if Harris will boost the ticket with Black voters.

I don't want to downplay Harris's Indian American roots. But Black voters are expected to account for about 13 percent of the expected 2020 electorate, a much bigger share than Asian Americans (5 percent). Black voters are also a particularly sizable and important bloc in key swing states such as Florida (13 percent), Michigan (13 percent), North Carolina (23 percent), Pennsylvania (11 percent) and Wisconsin (5 percent.) I am addressing Harris's potential appeal to Black voters specifically not because I think Black voters are likely to be particularly energized by a Black woman like Harris, but rather because much of the conversation around the vice presidential selection has implied that picking a Black person will create extra enthusiasm for the ticket with Black voters.

The percentage of Black voting-eligible people who cast ballots was significantly higher in 2008 (65 percent) and 2012 (66 percent), when there was a Black candidate on the ticket, compared to 2004 and 2016 (both around 60 percent) when there was not. Some political science research shows that Black people vote at higher rates when a Black candidate is on the ballot, although that finding is somewhat contested, and that research is about voting for a Black candidate at the top of the ticket, not a white candidate with a Black running mate.

So it's not a crazy idea that Harris might boost the ticket with Black voters. It has some empirical basis. But I think the stronger case, at least based on what we know right now, is that she won't have much of an effect in terms of Black voters.

Why not? First of all, while it happened in 2008 and 2012, it's just really hard for Democrats to get that much more support from Black voters, who even in elections like 2004 or 2016 vote at fairly high rates (significantly higher than Asian American or Hispanic voters) and overwhelmingly support Democratic candidates.

Second, it's not 2008 or 2012, when Black voters had the chance to elect and then reelect the first Black president. Third, Harris herself seems unlikely to particularly excite Black voters. This is not a random guess - Harris ran for president for much of last year and was not the preferred candidate of older Black voters in the Democratic primary (that was Biden) or younger Black voters (that was Sanders or Warren in earlier stages of the race). Obama, in contrast, had very strong Black support during his 2008 primary run, previewing what would happen in his two general election campaigns.

We don't know how Harris's selection affects the protest movement that has emerged since the police killing of George Floyd.

Another part of the discourse has been that the selection of a Black woman became more necessary in the wake of the national protests around racial inequality over the last several months. But it's not clear the Black Lives Matter activists organizing these protests view a Black woman being picked as vice president as a major priority in terms of addressing racial inequality in America (as opposed to, for example, reducing spending on policing.) And some more liberal Black people are wary of Harris because they felt she was too punitive toward people who committed minor crimes like truancy when she was the district attorney in San Francisco and then the attorney general of California.

I would emphasize all of this is a bit unknown. Maybe Harris's choice will be applauded by BLM activists; it's just not a given.

We don't know if Harris is now the most-likely Democratic nominee in 2024.

If Biden and Harris lose, Harris's presidential ambitions are likely over. (Just ask Joe Lieberman, John Edwards, Sarah Palin, Paul Ryan and Tim Kaine.) But if they win, remember that Biden will enter office at age 78 and that in March he described himself as a "bridge" figure who will help usher in the Democratic Party's next generation of leaders.

So is Harris being set up to be the Democratic front-runner and likely nominee in 2024? I don't know. And honestly, Biden and Harris may not know either. But this matters, for two big reasons. First, in the 2020 election, expect Trump and his campaign, who have had a hard time casting Biden as an extremist or a radical, to make attacks on Harris with sexist and racist undertones, cast her as an ultra-liberal Californian out of touch with Middle American values and suggest that voting for Biden in November means that Harris will be running the country for 12 years.

Second, if Harris is vice president, she would need to make sure she is ready to run for president in 2024 - in case Biden either doesn't want to run for a second term or isn't up to it - while also ensuring she isn't portrayed by the media as constantly planning her 2024 campaign, which would likely to irritate Biden and other Democrats.

Biden made the predictable pick (in fact, we kind of predicted it in March). Harris is more liberal than Biden but not Sanders/Warren-left, and she is an Asian and Black woman in a party that always wants to show it cares about traditionally marginalized groups and probably feels the need to showcase its racial diversity even more in the wake of the Floyd protests. So Biden choosing someone with Harris's political, biographical and demographic attributes makes a lot of sense.

The biggest unknowns are around Harris herself and her electoral skills. Harris is a good politician based on these facts alone: She was elected senator in the nation's most populous state and in a country with a lot of race and gender discrimation, Harris is the second Black woman ever elected to the Senate, and arguably the first to be a serious presidential candidate. That said, it's still not clear if she is a particularly strong politician on the national stage, and therefore if she will be that helpful for Democrats in the No. 2 slot in 2020 or as the main candidate in a future presidential election.

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