The Republican establishment breathed a huge sigh of relief last fall when GOP presidential hopeful Donald Trump signed a loyalty pledge binding him to endorse the GOP nominee, whomever it is, and precluding a third-party run. At the time, a close associate said the candidate felt he’d been “treated fairly” by the party, which had been Trump’s stated criterion for deciding to sign the pledge. A lot of things have changed since then — including both the relationship between Trump and the party and the candidate’s commitment to the pledge, apparently. As the New York Post reports, the GOP frontrunner...