On Tuesday morning US time, US Vice President Kamala Harris called Minnesota Governor Tim Walz to ask him to be her running mate in her campaign for the presidency. After telling Harris he “would be honoured”, the first thing Walz observed was “the joy that you’re bringing back to the country, the enthusiasm that’s out there”. Harris’ choice of Walz confirms and leans into this extraordinary vibe shift in American politics. In only a fortnight, the campaign has been flipped on its head. The choice of vice-presidential running mate is often understood as relatively unimportant in the grand scheme of…