Former GOP presidential candidate Mitt Romney listed a half-dozen Republican candidates he said could qualify as mainstream conservatives his party could live with in a general election — and said he didn’t think Donald J. Trump was one of those. Mr. Romney, a two-time candidate who was the party’s standard-bearer in 2012, ticked off New Jersey Gov. Chris Christie, Ohio Gov. John Kasich, former Florida Gov. Jeb Bush, Sen. Marco Rubio of Florida, Sen. Lindsey Graham of South Carolina and businesswoman Carly Fiorina as viable options. -snip- Mr. Romney did not mention retired neurosurgeon Ben Carson, who along with Mr....