Greg Sargent: “There are 1,000 convoluted theories floating around on social media that purport to explain why Donald Trump won the presidential race. But some Democrats and people on Kamala Harris’s campaign have concluded that one very significant reason is a straightforward one: It proved disturbingly difficult to persuade undecided voters that Trump had been a bad president.” “Internal testing in all the battleground states over the course of many months yielded a result that unnerved the campaign, according to a senior Harris campaign operative who has seen the data.