If you look over the numbers 277 Republican held seats is pretty much impossible, you'd be talking about winning around 15-20 D+4 or above seats. There aren't that many D+1 or D+2 seats in the country.
I think Republicans already hold most of the D+1/D+2 seats that exist.
2016-2020 will be similar time period to 1976-1980, whoever wins in 2016 will cause their party to be crushed by a realignment in 2020.
Why?
For many reasons no economic expansion in American history has lasted more then 10 years so we can expect a recession by 2018. Also we will have a major foreign policy troubles in 2020, for the republicans they will invade a country and like Iraq get bogged down there, with Democrats ISIS will likely grow, Russia makes more trouble and possibly Iran.
If a Democrat is in office Democrats will get all the Blame for any trouble due to holding the white house for 12 years and like Bush in 1992 they cant blame Congress. If Republicans win and things collapse on them like it did for Carter they will get the Blame as they hold all three branches for government.