In 2018, at the midpoint of the Trump Presidency, the journalist and historian Jon Meacham wrote a book called “The Soul of America,” warning of the gravity of Trump’s threat to democracy. This was hardly a unique point of view, but Meacham’s particular way of putting things, steeped in a critical reverence for American history, hit home with one reader in particular: Joe Biden. In the years since, Meacham became an informal adviser to Biden, helping him recently with the State of the Union address. Meacham, who has written biographies of Thomas Jefferson, Andrew Jackson, George H. W. Bush, John Lewis, and, now, Abraham Lincoln, reflects on the vulnerability of American democracy in the current moment, with an overt autocrat as the leading Republican contender for the next Presidential election. “Having a dictatorial figure is not new either in human experience or American history. What is new is that so many people are willing to suspend their better judgment to support him,” he says. “I am flummoxed to some extent at the durability of partisan feeling.”