It's two years into President Trump's first term. From 8 years ago, we expect the Democrats to have the advantage this year, especially since Trump is not highly approved of. In late 2017 and early 2018, that would be all I had to say, as there were 20 to 30 point switches from Republicans to Democrats. But that wave seems to have ended. Times have changed: Trump's approval rating is a slow, upward trend now, and the generic congressional polls are far closer than even a month ago. If I had to say one thing about this, it's that the Democrats were too rash and impulsive in their hate for Trump, and as a result, the Blue Tsunami crashed ashore a year early.
On the sidebar, you will find House, Senate, and Governor's Mansions predictions. In the house, all 435 seats are up. In the Senate, 35 seats are up. And 36 Governor's Mansions are open.
Edit 18 Nov: Though the Mississippi Runoff hasn't happened yet, the election has been a total disappointment to both sides. Yet, both seem quite pleased.
Democrats have gained (allegedly) the most seats in the House since 1974 (that's Post-Watergate), but lost a net 2 seats in the Senate. They also, somehow, held on to CT, where the state's wave elections are off-sync to the nation. They got Kansas and Wisconsin, but failed to get Ohio, Iowa, or Florida.
Republicans mixed their strategy up in late September for the Senate. Before then, they were working in the Upper Midwest, but then they shifted their focus to red states after the Kavanaugh debacle. It worked. They got North Dakota, Missouri, and Indiana. They also got Florida, as a sweet bonus. House Democrats, for the most part, also underperformed. But this election was a total mess.
I have a bit of a hitch that there was voter fraud in multiple states this election. Votes don't usually go in so slow that it takes a full week or more to get them fully counted, and then find more. There has been multiple reports of people randomly finding ballots on the street days after the election, and getting turned in, thus changing the result. In all but two cases, it has benefited the Democrats, allowing them to pick up seven house districts, one senate seat (almost two), and one governor at least two days after the election.
I still fully recognize that Democrats took the house. I had seen that the morning after the election. I am saying that some results (from 228->235) are slightly questionable.