After weeks in which the fate of the Senate simmered at nearly even odds of flipping for Republicans or remaining in Democrats' control, the outlook has shifted dramatically in the Democrats' favor. The incumbent party now has an 80 percent chance of retaining its majority, according to the Signal's prediction model.

The break is largely due to critical races in Missouri, Wisconsin, Massachusetts and Virginia, all of which have favored the Republican at some point in the past month and now favor the Democratic candidate.

The cards appeared heavily stacked in the Republicans' favor going in to this election season. The Democrats control the Senate, but they have only 30 returning senators to the Republicans' 37 returning senators among the two-thirds of the chamber not up for re-election this cycle. That, combined with strong anti-incumbent currents in the electorate, presented an uphill battle for the Democratic Party.

The trouble for the Republicans began with Missouri with Republican nominee Todd Akin's comments parsing different types of rape. This had been a likely pickup for Republicans, given the unpopularity of Democratic incumbent Claire McCaskill. The prediction markets wavered on this race given the uncertainty as to whether Akin would be forced to withdraw, but have moved in McCaskill's favor as the likelihood that Akin will be replaced has drifted to zero.

That shift alone was enough to throw the fate of the Senate into turmoil, because it added one more variable to an already complicated list of scenarios. The front-runner in Maine, independent Angus King, has not said which party he will caucus with if elected. In the event of a tie, control will go to whichever party wins the White House.

Next, we have a roller coaster in Wisconsin. Democratic candidate Tammy Baldwin initially looked strong against a field of Republican possibilities. Then, the strongest of those, former Gov. Tommy Thompson, won the primary. For a while, he held a commanding lead, dominating the polling after his primary win. A series of polls in mid-September demonstrated a massive surge for Baldwin, however, and the markets have responded accordingly.

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The Massachusetts senatorial race, the most anticipated senatorial contest of the election, has been close all along. The incumbent, Republican Scott Brown, faces a formidable opponent in Elizabeth Warren. The state is heavily Democratic, and that party identification may prove to be the tiebreaker for Warren, who has pulled ahead in recent polling and is now 61.6 percent likely to take the Republican-held seat.

The Virginia senatorial race pits former Sen. and former Gov. George Allen against former Gov. Tim Kaine. Kaine, the Democrat, has pulled ahead in this race in concert with Obama's odds in the Old Dominion. Kaine is now 66.8 percent likely to carry the open Democratic seat that retiring Sen. Jim Webb captured from Allen six years ago. Obama has a 73.9 percent chance to repeat his narrow win in the state in 2008.

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