A generational and structural shift is decoupling Black identity from Democratic Party loyalty, transforming a once-reliable voting bloc into a cohort of "political free agents" that the GOP is uniquely positioned to exploit. Why it matters: Even modest GOP gains — combined with weakening party loyalty — could make a big difference in close elections in a post- Voting Rights Act world. President Trump is making gains with Black voters despite posting racist videos , using racist rhetoric and advancing policies critics say erase slavery history and weaken voting rights. An Axios review of recent data shows breaks in the strong Black support for Democrats going back to John F. Kennedy's 1960 presidential run and Barack Obama's historic 2008 win. Zoom in: Trump is benefiting less from persuasion and more from a changing electorate. It's younger, more diverse and less tied to traditional party loyalties, said Theodore Johnson, a senior adviser at New America.…