Well, the question is about Congress. I don't think anyone has suggested 2018 yet- scenario would be Republicans sweep everything in 2016 and then a big enough backlash against their agenda to flip the house. Not sure how the senate could possibly flip R to D in 2018, though. What would they pick up other than NV?
It all depends on the environment - West Virginia would have a strong chance of flipping if Manchin opts to run for Governor, and Colorado could also be a potential. Republicans certainly have a lot more to defend, but I think they'll also be starting out with a good advantage in a lot of states (Ohio, New Hampshire, Florida, North Carolina, Missouri, Arizona). The only states that begin automatically as a toss-up are Illinois and Wisconsin, as well as Pennsylvania according to who you talk to.
Obviously a lot of hypotheticals - I didn't think Republicans had much chance of taking the Senate in 2014 a year ago but it is looking pretty good now.
Well, Republicans won only a single senate race in a state carried by Obama in both 2008 and 2012. In 2016, there are 7 Republican seats up in Obama 2012 states (but some were very narrow), so if Democrats win the presidential election, I would fully expect D+4 or so in the senate. But if a Republican won the presidential election by enough to swing the PA/WI/NH class of states, I could see a break-even or R+1 scenario. Now if Hillary ran and won, say Obama 2008+GA+AZ+MO, then you could be looking at something more like D+8. In short, it would shock me if Republicans held onto the Senate without picking up the White House.