With the unrelenting focus on every aspect of the fight for the White House, it's easy to forget that the Senate is also up for grabs on Election Day. In fact, it could be the presidential election that determines the upper body's control: If the Republicans win a net total of three seats, the Senate will be divided 50-50. In that case, control will go to the party that wins the presidency. (If you need an eight-grade civics refresher, this is because one of the vice president's few official duties is to break a tie.)
Currently, the Democrats control 54 votes: 51 Democratic senators, two independent senators who caucus with them, and one vice-president. But the way the dice fell does not favor them: Democrats control 23 of the 33 Senate seats that are up this cycle, giving them much more territory to defend and many fewer opportunities to pick up seats. Of those 23 races, seven are open seats (i.e., the Democratic caucusing member is leaving the senate), while four of the 10 Republican seats are open.
The prediction markets are aware of this, of course, which is why the odds that Democrats will retain their majority currently rest at 41.5 percent. That's a major improvement for them since the beginning of the year, when their odds clocked in at 25 percent.