Former President Donald J. Trump said late Friday that he agreed with Fox News to debate Vice President Kamala Harris on Sept. 4. If it happens, it would be the second presidential debate this election cycle and the first between Mr. Trump and the new Democratic candidate.According to Mr. Trump’s post on his social media site, Truth Social, the debate would take place at a to-be-determined location in Pennsylvania, one of the most consequential battleground states. Fox News anchors Bret Baier and Martha MacCallum would moderate.It was unclear early Saturday whether Ms. Harris had agreed to the debate and its...