WASHINGTON — Mitt Romney sought to use the coveted endorsement of Jeb Bush on Wednesday to amplify his call for Republicans to rally behind his candidacy and get on with the mission of ousting President Obama............... But if the endorsement held the potential to further choke off the oxygen to Mr. Santorum’s insurgent candidacy, the Romney campaign inadvertently gave Mr. Santorum a new supply when a senior adviser went on CNN and seemed to suggest that Mr. Romney’s conservative positions in the primary season could change like an Etch a Sketch drawing. “I think you hit a reset button for...