Washington, D.C. — It was billed as an attempt to win new supporters from the ranks of congressional Republicans. But after Donald Trump convened a handful of lawmakers and a number of party luminaries for lunch on Monday, the sole attendee not already backing Trump’s candidacy emerged as uncommitted as he’d gone in. Held in D.C.’s high-powered Jones Day law firm, the confab was supposed to be a congressional outreach effort, but only one attendee — Senator Tom Cotton — was neutral on Trump’s candidacy. The others — including Senator Jeff Sessions and Representatives Chris Collins, Scott DesJarlais, Renee Ellmers,...