A former CIA case officer who has served as top policy aide to House Republicans is launching an independent campaign for the presidency on Monday with the backing of veteran GOP strategists and donors determined to block Donald Trump of getting anywhere near the White House. Feeding off mounting discontent within GOP ranks over Trump, Evan McMullin — who is resigning today as chief policy director for the House Republican Conference — will announce his campaign with an open “Letter to America” aimed at rallying independents and conservatives to his cause. “It’s never too late to do the right thing,...