You might be surprised to hear this, but of U.S. Senate races that have been called as of this morning, Democrats (or independents who caucus with Democrats) had won 22, Republicans had won nine, and Republicans had slight leads in three others. Even if you assign those three seats to the Repubs, Democrats had won 22 out of 34 (or 65 percent) of the 2018 Senate races.
That gets in the way of the oversimplified analysis point that the good Democratic night in House races was offset by the good Republican night in Senate races. Democrats actually won a significant majority of the races for seats in both houses.
Nonetheless, Republicans went from a bare one-vote majority in the Senate to a significantly more comfortable 54-46. (That will also end their heavy reliance on their two or three least conservative members, like Susan Collins of Maine and Lisa Murkowski of Alaska, to pass a bill through the Senate – although passing a bill through the Senate also declines in importance since Democrats now control the House, so nothing can become law without some Democratic votes).
But to get back to the numbers at the top, Democrats won 65 percent of the Senate races, yet lost ground. That, obviously is the result of an oddity (compared to most other democracies in the world) of the U.S. system: the staggered six-year terms of senators, which results in only about a third of the 100 Senate seats being up every two years. We’re so used to it that it may not look odd to us, but it is and, in this particular case, it led to the fairly flukey result that Democrats lost ground in the Senate while winning most of the races.