Chicago’s public school teachers will be asked again this week to authorize a strike in case they decide to walk out of Chicago Public Schools later this fall.

Most teachers, aides and social workers in the Chicago Teachers Union are being asked to vote Wednesday morning before school, but any schools that don’t get everyone who’s eligible can also try again Thursday and Friday if necessary, according to the union, which did not release a sample ballot. Results won’t be immediately available, the union has said.

The vote is one of many steps required for teachers to legally strike. They need at least 75 percent of their membership to agree.

They also need to give a state board 10 days notice before walking out — and they haven’t yet done that. Unless the union calls for an emergency meeting, its governing board next meets on Oct. 5. If the House of Delegates votes that night to set a strike date, union members could walk out as early as Monday, Oct. 17.

CTU President Karen Lewis has said she doesn’t want her members to walk, but she said that they also won’t work another school year without a contract. Their last one expired in June 2015.

Officials from the union and the Board of Education continue their negotiations, which began almost two years ago, but have mostly refrained from the kind of hostile talk that lead up to the 2012 teachers strike.

The CTU held an earlier election in December in which 88 percent of total membership agreed to authorize a future strike — but CPS contested the timing of that vote, saying it happened too soon. CTU officers have said the repeat vote should quash any concerns.

School-based employees will vote at their schools after showing a CTU card or pay stub showing deduction of dues. Citywide employees who float among buildings can vote at any school or at the CTU office, 222 Merchandise Mart, between 8 a.m. and 6 p.m. Members can vote at a number of drop-off locations between 3 p.m. and 5:30 p.m. listed on the union’s website, which also read, “Look for a CTU-marked car in the school parking lot.”

Labor expert and University of Illinois at Urbana Champaign Professor Robert Bruno said the vote doesn’t guarantee teachers will walk picket lines. Instead, he called it a “signaling effect to the school district.”

“You’re running out of time,” he said it signified. “We’re going to give you a warning. . . . We want to have control over how a final settlement is going to occur, and by taking the strike vote and setting the strike date, that clock’s going to tick loud.”