In the long American drama of political identity, Senator John Fetterman of Pennsylvania offered a rare and complicated scene this week: a Democrat stepping onto a Republican stage, not to cross over, but to honor a friendship across the aisle. His appearance at the GOP midterm convention in Dallas to introduce Sen. Dave McCormick sparked immediate speculation about a party switch, but Fetterman moved quickly to reaffirm what he called a lifelong Democratic allegiance — even as his relationship with that party has grown visibly strained. It is a moment that speaks to the widening tension betwe