President Trump’s image of political invincibility, critical to his influence with congressional Republicans and grassroots conservatives, is under assault amid a sweltering summer of gloomy public opinion polls.

Trump has trailed presumptive Democratic nominee Joe Biden for months — nationally and in an expanding array of battleground states. A sustained struggle like that would test any presidential contender and raise questions about his or her campaign. But for Trump, the spate of discouraging data presents deeper challenges. It makes the president look vulnerable, undermining his brand as a strong leader, which appeals to the GOP base and keeps congressional Republicans in line.