[Updated] An elections watchdog group is urging Philadelphia officials to quickly schedule a special election for a Council seat that could soon be vacated.

Gov. Tom Corbett announced Friday he's nominating Councilman at-large Bill Green to lead the School Reform Commission, the five-member panel overseeing the city's school district. Green's position must be confirmed by the Pennsylvania Senate.

It is up to Council President Darrell Clarke to determine whether and when to hold a special election, according to several election law experts. Ward leaders will choose each party's nominee if a special election is held, they said.

Ellen Kaplan, vice president of the elections watchdog Committee of Seventy, said Clarke should set the special election for May 20, the same day as the gubernatorial primary, assuming the Senate's confirmation process is expeditious.

"There's no reason to wait," she said. "We would urge Council President Clarke to set the special election as soon as possible so that the voters can have their full complement of at-large Council people representing them."

Clarke said he has not yet decided whether to schedule a special election.

"I think it's premature to make that determination at this point," he said. "We'll kind of let the whole nomination and confirmation process play out, and then we'll talk with the [Council] members to determine what the next steps are."

If the Senate confirms Green, election law attorney Adam Bonin said Clarke may let the seat remain vacant for some time. It took more than a year for a special election to be held after then-Councilman at-large David Cohen died in October 2005. Anna Verna, the Council President at the time, set the election for November 2006.

"I have no reason to believe that this vacancy is going to be filled soon," said Bonin. "Given that it's at an-large seat, such that there is no district of constituents that's directly affected, he may well decide to forgo a special election for some time."

Bonin also raised questions about whether other legal experts correctly interpreted the law on special elections. He said a 1955 decision by the Philadelphia Court of Common Pleas suggests that Clarke has no choice but to set an election date.

Conversely, veteran election attorney Gregory Harvey said there is case law showing that Clarke can choose whether to schedule a special election.

All of Philadelphia's 17 Council members are up for election in 2015.