I realize this is a discussion for the US politics thread...
If you look at just the Senate elections in 2018 - there are only 33 seats up for election.
Of those 23 are seats currently held by Democrats and only 8 are Republican.
These are the States the 8 Republican seats are in - Texas, Wyoming, Utah, Tennessee, Nebraska, Mississippi, Arizona and Nevada. Of those, Only Heller in Nevada is a toss up, all the others are safe Republican seats.
Whereas the Democrats have 3 toss ups - Heitkamp in North Dakota, McCaskill in Missouri, Donnelly in Indiana) and of course that complete and utter wanker Manchin in West Virginia.
That's before you even get to the House (where all the seats are up for election), but...Democrats sucking on balls for years at the local level have allowed Republicans to stack things very strongly in their favor in lots of districts.