The Chicago Teachers Union (CTU) began a strike authorization vote Wednesday.
The vote runs through Friday.
CTU members authorized a strike in December, but union leaders say a second vote would allow them to ensure the legality of holding a strike this fall.
Under state law, CTU would need authorization from 75 percent of its members to strike.
A 10-day notice is also required from the union before teachers can hit the picket lines. Mid-October is the earliest the CTU could go on strike.
If a walkout happens, it would be the second strike staged by the CTU since 2012.
The union is still in negotiations with the school district over a new labor contract. CTU's contract with the school district expired in June 2015.
The two sides have been at odds over benefits, pensions and other items.