Former President Barack Obama’s campaign mastermind is joining Vice President Kamala Harris’s presidential campaign as a senior adviser. David Plouffe developed a reputation as a campaign guru by guiding Obama from being a little-known junior senator from Illinois to the White House in one of the most odds-defying and historical campaigns in American political history. He served as a top adviser in Obama’s White House, essentially leading Obama’s 2012 from his perch in the administration. He will now cast his lot with Harris, the latest Democrat power player to do so after countless party bigwigs had cooled on President Joe...