As their midterm picture darkens, and gas and diesel prices soar, some Republicans who backed the war for months are now changing their tune. As President Trump warned in a speech to the United Nations General Assembly in recent days that he might “annihilate” Iran, a growing number of Republicans were scrambling to find distance from a costly war that has swelled into a midterm crisis for the party.The split screen showed how Mr. Trump and his party’s battleground candidates are increasingly at odds as the November elections approach. With no clear resolution in sight, the war has sent the...