The biggest electoral question of the year is undoubtedly who becomes the next president. But just after that follows the issue of whether the Senate majority will flip again. Republicans took the chamber with a 54-46 seat majority in the 2014 midterm elections. Keeping that lead in 2016, however, will prove a more difficult task.

According to The Huffington Posts Senate model, which relies on the polls aggregated in HuffPost Pollster charts , theres a 55 percent chance that the Senate will swing completely over and a 23 percent chance that its tied at 50 seats for each party. That means theres a 78 percent chance that the Democratic Party could get 50 or more seats.

Part of the Democrats advantage comes from holding more safe seats in the Senate than the Republicans; they have 36 seats that arent up for election, compared to the GOPs 30 safe seats. But some Republican candidates are underperforming, possibly thanks to Donald Trump at the top of the ticket .

Based on projections for the presidential race, a tie would still be good news for Democrats. Most models give the Democratic ticket of Hillary Clinton and Sen. Tim Kaine (D-Va.) at least an 80 percent chance of winning the presidency. That would make Kaine the tie-breaking vote in the Senate as the vice president, shifting Democrats back into the majority by a 51-50 split. Of course, if the presidential election goes the other way, Republicans would have the upper hand.

The model also produces probabilities for each individual Senate race where polling is available, and approximates the state of the race where there isnt any or enough polling to run the model.