CHICAGO — Teachers in the nation’s third-largest school district are casting ballots on whether they support a strike.

Members of the Chicago Teachers Union began voting for a strike authorization on Wednesday. Voting lasts through Friday. Union officials say the results won’t be available right away.

The union needs 75 percent support, which is expected. A similar vote in December garnered support from roughly 88 percent of the voting members. Union members say the second vote offers legal cover.

If the union goes on strike, they have to give the district 10 days of notice. The earliest a walkout could take place is mid-October.

The contract expired in June 2015. Negotiations with Chicago Public Schools officials have focused on cost-of-living raises, pension contributions and health care.

The union represents over 25,000 members.