The six-year terms in the Senate produce a curious electoral quirk: The party that controls the chamber going into the election is not necessarily the one in the best position to control it coming out of the election, even in a neutral political environment. This is because the 33 or 34 seats up for election each cycle are usually unevenly divided between the parties. In 2006, for example, Democrats took control of the Senate by a hair, even though 40 of the 55 Republican seats were not up for election that year.

The Democrats currently control 53 of 100 seats in the Senate. Nearly half of those—23—are up for re-election this year, while the Republicans are defending only 10 seats. That fact, combined with an electorate none too pleased with incumbents, made for a grim picture for the majority party at the start of this cycle.

And yet, the Democrats now have a 75 percent likelihood of controlling the next Senate, possibly by a comfortable margin.

In early September, we saw four critical races shifted to the Democratic column: Missouri, Wisconsin, Massachusetts and Virginia. Today, the Democrat is up in all four races, though three of the four remain competitive. Now, we're seeing five other races once considered safe for the Republican showing signs of equivocation: Nevada, North Dakota, Indiana, Arizona and Montana.



We expect the underdog to win in one or two of these nine unresolved races. If Democrats can claim four, they will control 51 seats in the next Senate, meaning they will maintain a majority even if Mitt Romney wins the White House. The more competitive races there are, the more the possible outcomes multiply. The Democrats could end up with a comfortable margin, or easily lose the chamber.

Follow the state-by-state and overall senatorial predictions in real time with PredictWise.com.

David Rothschild has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. Follow him on Twitter @DavMicRot