Alan Abramowitz: “Since World War II, only one incumbent, Jimmy Carter in 1980, has lost his bid for a second term when his party had reclaimed the White House four years earlier. According to my own ‘Time for Change’ forecasting model, first-term incumbents like Trump enjoy a significant advantage beyond what their approval ratings and economic conditions would predict. However, there are strong reasons to suspect that Donald Trump may not benefit from the same incumbency advantage that previous first-term presidents have enjoyed.”
“According to recent research on congressional elections, the advantage of incumbency has declined sharply in recent years as a result of growing partisan polarization. Gary Jacobson of the University of California, San Diego has shown that voters have become increasingly reluctant to cross party lines to support incumbents based on their voting records or constituency service. The same logic may well apply at the presidential level, especially with an incumbent like Trump whose electoral strategy is based on reinforcing partisan divisions among the public.”

