Senate Republicans had gathered for one of their regular private lunches last Wednesday when Rand Paul commanded their attention. As Senators sipped diet sodas and grazed on sandwiches, the Kentucky lawmaker went through a slide deck with charts and statistics in service of a bold proposal: come out against President Trump’s tariffs. The arguments in Paul's presentation, which has not been previously reported, were not a surprise to his audience. One of the most prominent libertarians on Capitol Hill, he’s a fierce proponent of free trade. But his attempt to corral colleagues against the Trump trade agenda was seen as...