Roland Burris was appointed to that position.
On Thursday, a federal appeals court ruled that his term will expire on the third of November, which means we voters will be asked to vote twice for the Senate seat on Election Day.
One vote will fill the 60-day period between when Burris' time is up and the swearing in of new senators in January, while the second will be to select the person who actually serves the full six-year term.
Making matters more complicated, Burris, the man who once seemed content to quietly retire, says he intends to fight to stay in the Senate.
"I'll tell you one thing: If there's a special election, I'm announcing my candidacy right now," Burris said last Friday. "I will run!"
Burris says no one is going to take away his final few months in the United States Senate.
"I have great respect for the federal courts, but what a mess," said Senator Dick Durbin, Illinois' other senator.
The appeals court ruling concludes Burris' appointment by then-Governor Rod Blagojevich was only temporary, meant to be a placeholder between the time Barack Obama left the seat to move in to the White House and when voters could next get to the polls.
A new election, the court says, is the end of that temporary term.
"At this point, it's in turmoil, uncertainty, and I'm afraid it's going to create a lot of confusion for voters," said Durbin.
If they are stuck with a second, simultaneous election, party bosses would prefer for the candidates in both contests to be the same: Democrat Alexi Giannoulias and Republican Mark Kirk.
However, a judge will be the ultimate decider of who will be on the ballot to fill the short two-month term.
"The voters of our state are not likely to see ads from those candidates, and it's going to be a mystery to them as they go in and try to figure out what this ballot means," said Durbin.
Former Chicago Alderman Martin Oberman filed suit seeking the dual election.
"In this country we elect our people," said Oberman. "Yes, you can appoint people on a short-term basis, but they shouldn't sit there for two years."
A top aide to Roland Burris says the senator believes replacing him in November would disenfranchise voters, which is why he'll fight to stay in the Senate.
Next week, there will be a court hearing to determine who will be on the ballot to serve the two-month term.
There will be an extra cost to the state associated with a second, simultaneous election, but it won't be as high as if this was a completely stand-alone contest.
