There was a time when Republicans used to refer to themselves, proudly, as “the party of Lincoln.” But you don’t hear that line much these days. Why? The main answer, presumably, lies in the G.O.P.’s decision, long ago, to seek votes from Southerners angered by the end of legal segregation. With the old Confederacy now the heart of the Republican base, boasting about the party’s Civil War-era legacy is no longer advisable.