WASHINGTON — A mad scramble is underway to introduce Vice President Kamala Harris to Americans who know little about her, with both campaigns vying to leave a vivid impression that sinks in before voters start casting ballots this fall. In the 11-week sprint to the election, Harris’ campaign is pouring tens of millions of dollars into an ad blitz meant to humanize a candidate whom the electorate may know only as the understudy to Joe Biden. As Democrats assemble in Chicago, Harris’ challenge is to persuade voters that the story she’s telling about her life and vision rings true, and...