After months of speculation and a seemingly endless cast of candidates through the revolving door, we now know what should have long ago been settled.

The former Vice President and presumptive Democratic nominee for president, Joe Biden, has announced that his running mate will be the US senator from California.

This is very good news on multiple levels. For starters --

Electability:

If you want Biden elected -- and especially if you want President Donald Trump defeated at all costs -- Harris is the only viable running mate to help take Biden across the finish line. Nearly every other candidate had major baggage or alienating qualities.

For example, Massachusetts Sen. Elizabeth Warren, a favorite punching bag of Trump and his supporters, is deeply distrusted by the right, and her national approval was underwater as of May. Former Obama national security adviser Susan Rice has problems with both the far-right and the far-left for her controversial foreign policy record. Michigan Gov. Gretchen Whitmer has little national name recognition and doesn't add racial diversity to Biden's ticket. Stacey Abrams, the former Georgia state representative who ran unsuccessfully for governor, is seen by some as too green. Others, too, had issues.

Harris, who ran for president herself, has already been vetted. She survived the primary relatively unscathed and emerged the top candidate to hop on Biden's ticket. In June, a panel of Democratic primary voters were asked to name their preferred choice for a female running mate for Biden -- a plurality picked Harris.

Though some progressives take issue with her criminal justice record, she worked hard to answer them during the primary. Her platform aligns fairly closely with Biden's, and there does not appear to be any major gaps she'll have to explain or contort to defend.

Harris wants to be president. Occasionally the ambitions of a vice president can get in the way of a seamless working partnership. But in this case, her positioning of herself would be a good thing, because she very well might be president someday. Biden will turn 78 on Nov. 20, and, if he wins, it's reasonable to consider he could be succeeded at some point by his vice president. It's important to have someone who's not only ready for that role, but who has envisioned how she would do it. With Biden at the helm, he'll set the pace, but his administration -- and the nation -- can rest easy that Harris isn't just a plus-one. She's ready to go.

Authenticity:

With Harris, Biden has put his money where his mouth is. It's one thing to say you care about ending racism, it's another to put a woman on the ticket who will make it her priority. If he truly empowers her to do just that, to have a voice on those issues that even overpowers and outshines his own, it could go a long way toward reassuring many Americans on the left and the right, young and old, White and Black, that an older White guy is truly interested in helping to usher in a new era of racial justice.

Crossover appeal:

Finally, it's Harris's po