But, far sooner than they expected, growing numbers of prominent Republicans are debating how far to go in revealing that they won’t back his reelection — or might even vote for Joe Biden, the presumptive Democratic nominee. They’re feeling a fresh urgency because of Trump’s incendiary response to the protests of police brutality, atop his mishandling of the coronavirus pandemic, according to people who spoke on the condition of anonymity to disclose private discussions.