The Washington Post editorial board had a mostly glowing assessment of Vice President Kamala Harris’ newly-selected running mate, Minnesota Gov. Tim Walz — and yet, they warned, there is a vulnerability the campaign needs to get ahead of. “Mr. Walz is a former teacher who comes off as an upper-Midwestern everyman on television,” wrote the board, noting that Harris’ selection “could be a play to widen her party’s polished coastal image — or an attempt to placate her party’s left wing. Most likely, it was both.” On one hand, the board wrote, Walz has a “history of winning over voters who don’t t…