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.