All we need to do is look at history. We have a president facing his second mid-term election. In the past nearly 100 years, only one president has enjoyed congressional seat gains in their sixth year--Bill Clinton in 1998.
Second-term losses for the president's party have occurred in five out of six of these type of elections since the end of World War II, averaging six Senate and 29 House seats. Six senate seats will flip the chamber, by a razor thin margin. Arkansas, Alaska, Louisiana, Montana, North Carolina, South Dakota, and West Virginia will net the Republicans seven seats and the upper chamber.