Election watchers say there's a significant chance the Senate will be evenly divided between Republicans and Democrats in 2017, a result that would seem to ensure another few years of gridlock in the upper chamber regardless of who wins the White House.
"It's certainly within the realm of possibility," according to Ron Faucheux, president of Clarus Research Group, which analyzes political polls.
Today, Senate Republicans control 54 seats while Democrats plus two Independents who vote with them hold 46 seats. Just about any Senate election year could lead to a tie, but 2016 offers some avenues for the even split.
"It's possible as it's always possible when the majority party has more vulnerable seats than the size of their majority," Faucheux told the Washington Examiner. "I count one vulnerable Democratic seat and seven to 10 vulnerable Republican seats. Mathematically, a net loss for the GOP of four seats, getting it to 50-50, is certainly within the realm of possibility."
Jennifer Duffy, senior editor at the non-partisan Cook Political Report, said the chance of a split Senate in January "is less than 50 percent," and notes the need for a likely run-off in the Louisiana Senate race, which means the outcome may not be final until after December 3.
The Senate could end up tied if the GOP loses five Republican seats and picks up Nevada, which is now held by Minority Leader Harry Reid, D-Nev., who is retiring. Polls show Rep. Joe Heck, the Republican, statistically tied with the state's Attorney General, Democrat Catherine Cortez Masto.
And Republicans now seem likely to lose a handful of seats.
Two Republican Senate seats that are particularly poised for a Democratic takeover are in Illinois and Wisconsin, where Sens. Mark Kirk and Ron Johnson are struggling in the polls against their Democratic opponents.
The RealClear Politics average puts former Sen. Russ Feingold, a Democrat, ahead of Jonson by an average of 11 points. In Illinois, Kirk trails Democratic Rep. Tammy Duckworth, 37-44, according to Duckworth's own polling conducted in early August.
In Indiana, former Democratic Sen. Evan Bayh is leading Republican Rep. Todd Young by a margin of 54-33 in the race to fill an open seat.
In other states, Republican incumbents are behind, but not by much. New Hampshire's Kelly Ayotte trails Democratic Gov. Maggie Hassan by an average of one point, a statistical dead heat, while in Pennsylvania, Democratic challenger Katie McGinty leads GOP incumbent Sen. Pat Toomey by an average of 3 points.
Outside of the vulnerable GOP seats in Illinois, Wisconsin, New Hampshire and Pennsylvania, recent polls show other competitive races favoring Republican incumbents.
Sen. Marco Rubio, Florida's Republican incumbent, is holding on to a 6-point lead over Rep. Patrick Murphy, his Democratic opponent, while Sen. Rob Portman holds a similar lead in Ohio over his challenger, former Democratic Gov. Ted Strickland.
If the election produces an evenly split Senate, the vice-president would serve as a tie-breaker, which means the majority would hinge on whether Republican Donald Trump or Democrat Hillary Clinton wins the White House.
But even so, a 50-50 split spells gridlock in a chamber that requires 60 votes to advance legislation. Republicans have found themselves unable to get to final votes with 54 seats in the Senate.
A 50-50 split would also require some kind of power-sharing deal between the two parties.
In 2001, for example, Democrats and Republicans agreed to evenly split staff budgets and, more importantly, committee assignments. It was an arrangement former Senate Parliamentarian Robert Dove described as "a mess," because neither party had the majority needed to pass anything out of committee.
University of Virginia political science professor Larry Sabato told the Examiner said the shape of the Senate races will depend on how Trump and Clinton fare in the next three months, since their status at the top of the ballot often impacts House and Senate races.
"As I look at the big picture, Democrats have a good chance to take control of the Senate," Sabato said. "A 50-50 split is just one possible outcome. I could see Democrats winning with a buffer of a seat or two just as easily."