Why did anyone think Joe Biden was ready for a 90-minute presidential debate? One reason is that a small handful of journalists, with left-leaning sympathies but very credible records, have interviewed him and those close to him in the White House and returned with reports that while he’s definitely older, he’s still pretty sharp and in charge. Two of the most prominent are Biden biographers Evan Osnos and Franklin Foer. I spoke to them Sunday as they reconciled their own portrayals with the...