(Bloomberg) -- Chicago Teachers Union members are voting over the next three days on a resolution to only work remotely, setting the stage for a possible strike in the third-largest U.S. district as more teachers are supposed to return to schools next week.

Rank and file union members will cast votes Thursday though Saturday on a resolution to “conduct remote work only,” from Jan. 25, the date kindergarten through eighth grade teachers are slated to report in person with about 70,000 of those students expected to return Feb. 1. More than 80% of the union’s 600-member House of Delegates late Wednesday voted in favor of the measure, sending the matter to its 25,000 members, including teachers and support staff, for a vote using electronic ballots.

“In the absence of an actual commitment on safety from CPS leadership, the best assurance we have for the safety of our students and school staff right now is to continue remote learning,” Jesse Sharkey, president of the union, said in a statement. “Only the mayor can force a strike, and if it comes to that, that’s her choice. We choose safety.”

Like districts across the country, Chicago Public Schools shuttered its 642 buildings in March when the pandemic hit, and the district has grappled with when and how to bring students back. On Thursday, President Joe Biden said reopening guidance will be given to schools. If the union refuses to return to schools, that would constitute “an illegal strike,” according to a letter Thursday to district staff from Matt Lyons, CPS’s chief talent officer.

This would mark the second strike in as many years for the union. In October 2019, during Lori Lightfoot’s first year as mayor, CTU launched a nearly two-week strike, its longest since 1987, to demand higher pay, more social workers and a nurse in every school.

Lightfoot’s office referred questions about the union’s vote this week to the school district.

District officials have met with the union more than 60 times to discuss reopening schools safely and are “committed to reaching a mutually-acceptable agreement,” Emily Bolton, a spokesperson for the district said in a statement. “Stripping tens of thousands of students of the opportunity for safe, in-person learning is not an option or a viable solution for families who have been planning to return since December.”

Educators will be eligible for Covid-19 vaccinations in Illinois on Jan. 25 as part of the state’s next phase, which includes individuals who are 65 and over as well essential workers. The city also will set up vaccinations in schools in mid February, according to Allison Arwady, commissioner of the Chicago Department of Public Health.

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