The relationship between President Trump and Senator Mitch McConnell, the majority leader, has disintegrated to the point that they have not spoken to each other in weeks, and Mr. McConnell has privately expressed uncertainty that Mr. Trump will be able to salvage his administration after a series of summer crises. What was once an uneasy governing alliance has curdled into a feud of mutual resentment and sometimes outright hostility, complicated by the position of Mr. McConnell’s wife, Elaine L. Chao, in Mr. Trump’s cabinet, according to more than a dozen people briefed on their imperiled partnership. Angry phone calls and...