A few days ago, Donald J. Trump said vice-presidential nominees have “virtually no impact” electorally.

By selecting Minnesota’s governor, Tim Walz, as her running mate on Tuesday, Kamala Harris put Mr. Trump one step closer to being proved right.

A few days ago, it wasn’t so obvious Mr. Trump’s claim was on safe footing. While it’s true that most vice-presidential nominees don’t have a major impact, they often help the ticket in the vice-presidential nominee’s home state. The effect might be only a percentage point or two, but Vice President Harris was considering an unusually strong slate of options from key states — and the polls are close enough for one or two points to be decisive.

In the end, she did not choose someone like Gov. Josh Shapiro of Pennsylvania or Senator Mark Kelly of Arizona. Instead, she selected someone from Minnesota, which is not a top-tier battleground.

Looking back, perhaps her decision shouldn’t be surprising. Out of all the picks this century — JD Vance, Ms. Harris, Mike Pence, Tim Kaine, Paul Ryan, Joe Biden, Sarah Palin, John Edwards, Joe Lieberman, Dick Cheney — only Mr. Kaine hailed from what was considered to be a core battleground state at the time, Virginia. Instead, most vice-presidential nominees are selected with other considerations in mind, like a desire to bring balance to the ticket, or to add a governing partner, unify a fractured party or help define the presidential candidate — whether by reinforcing a candidate’s message or by trying to defuse the opposition’s critique.

By most of those other criteria, Mr. Walz is a very typical selection. As a white man from the rural heartland, he brings balance to a ticket led by a nonwhite woman from the Bay Area. He is a well-liked governing partner, as a former member of the House and a governor in his second term. He’s a pretty mainstream Democrat who avoids an intraparty fight over Gaza and lets Ms. Harris keep riding the good vibes of the last two weeks.