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 eighth-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.
The likelihood of the Democrats successfully defending the Senate has increased dramatically since the new year, however, a jump that is largely attributable to one person: Olympia Snowe.
The Republican senator from Maine's surprise announcement that she was retiring turned a safe Republican hold to a likely Democratic gain. You can clearly see this point on the chart in late February.
At almost the exact same time as Snowe's retirement announcement, Bob Kerrey announced he would run for Senate in Nebraska, turning a highly likely Republican gain into a toss-up. Kerrey is a popular former senator but has been living in New York City for many years. Since entering the race, he has slipped back into being a long-shot candidate with about a 15 percent chance to defend the seat.
On May 8 the Republicans received another surprise when Dick Lugar lost the Indiana primary to Richard Mourdock, a tea party challenger. This moved another highly likely Republican race into play, where the Democrats now have a 35 percent likelihood of capturing the seat.
The Democrats, meanwhile, are 55.1 percent likely to maintain control of the vice presidential vote (i.e., Obama's likelihood of re-election). We can see some connection between the presidential and Senate odds—many of the same fundamental variables, like economic indicators, influence both types of elections. Yet with little movement over the year in the presidential race, there is little correlated movement to show between the two major outcomes.
Follow along in real time with PredictWise.
David Rothschild is an economist. He has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. Follow him on Twitter @DavMicRot. Want more? Visit The Signal, connect with us on Facebook, and follow us on Twitter.