In a development that could dramatically change the composition of the federal penitentiary system, Republicans are projected to pick up as many as seventy seats in prison, a leading incarceration expert said on Thursday. “Prognostication is an inexact science,” Davis Logsdon, who studies conviction rates of corrupt politicians for the University of Minnesota’s Guilt Project, said. “Having said that, if current indictment trends hold up, the Republicans could be flipping at least seventy key prison seats.” Logsdon broke down criminal cases against Republicans into likely convictions, likely acquittals, and toss-ups, and found that the G.O.P.’s path to the magic number of seventy new prison cells was “very doable.”