The Chicago Teachers Union voted Wednesday evening to approve a one-day “showdown” strike on April 1 — which will include a massive downtown rally — to bring attention to ongoing contract negotiations and the state’s education funding crisis.

“I think it’ll be a clear message. Based on the current organizing, we’ll shut this city down,” Tammie Vinson, a special education teacher at Oscar DePriest Elementary School said after the vote at the International Operating Engineers Hall.

But the vote — 486 in favor, 124 opposed — was hardly unanimous, and some teachers expressed doubts about the amorphous action after seeing the motive change over the past few weeks.

Still, CTU President Karen Lewis said the numbers show teachers are ready to fight.

“This house is ready and united,” Lewis said.

The April 1 action will include picketing in the morning and a downtown rally in front of the Thompson Center.

“Strikes are about withholding labor because the labor conditions have come to a point where they’re not tolerable. We’re working under conditions that seem to be extremely unfair to our people,” Lewis said.

Lewis said the “no” votes came from delegates who “were ready to go now” for a full-on strike. But some teachers who voted no had other concerns, including missing two days of pay on one paycheck from a furlough day they’ll take this Friday along with next Friday’s planned action.

Where students will spend the strike day is also a concern for some teachers, as is another day with no instruction for students about to start advanced placement testing in the coming weeks.

But Lewis said members are “very, very upset and want to do something serious.”

She said the one-day strike must send a message to Republican Gov. Bruce Rauner.

“For us to continue to be submissive to somebody who has an agenda and that has always been about getting rid of us is ridiculous,” Lewis said. “We will not be submissive. We will stand up.”

The 27,000 union members have been working without a new contract since June 30. Recently, CPS CEO Forrest Claypool imposed three unpaid furlough days on all CPS workers, the first one on Friday.

“April 1 would be an unfair labor day of action,” Lewis said in announcing the proposed strike earlier this month. “It’s a showdown.”

Initially, Lewis and the union were reacting to Claypool’s threat to halt a 7 percent pension payment CPS had been making for teachers and other CTU staffers since the ’80s. After Claypool agreed to leave the pension benefit in place, the union accused CPS of an unfair labor practice for stopping raises given for experience and ongoing education last year.

The inclusion of a large group of union members and activists and the lack of specific contract demands in the one-day strike led some Southwest Side elementary teachers to balk at leaving their students on a scheduled school day, a source told the Sun-Times.

Another South Side high school teacher who didn’t want his name printed said neither he nor his students could afford to lose the school day.

“This process is plainly showing the union is not united,” he said.

On Wednesday evening, Claypool reiterated that the schools need teachers in the classrooms and their leaders at the bargaining table — especially so both sides can present a united front to their mutual opponent.

“We’re particularly disappointed that the CTU leadership has given Governor Rauner more ammunition in his misguided attempt to bankrupt and take over Chicago Public Schools,” Claypool said.

CPS lawyers say a legal strike can’t happen until mid-May after a state-mandated fact-finding process wraps up. Claypool has threatened that teachers or other CTU members who call off on April 1 without proof of a medical issue or true emergency will not be paid. He has promised contingency plans for the one-day action.

CPS and CTU’s leadership agreed on a contract proposal in January but the union’s 40-member big bargaining team rejected it, saying it left too many loopholes and included no new revenue so raises were at the expense of older teachers pressured to retire.

The school system has struggled all year to balance its budget and borrowed $725 million in bonds at extremely high interest rates to keep school doors open for the remainder of the year. It also has laid off hundreds of workers.

The financially beleaguered Chicago Public School system is still waiting for financial help from Springfield — help that isn’t likely to turn up anytime soon.

And the governor despised by public sector labor unions has launched a financial investigation of CPS he claims could justify a state takeover of the district.

Hours before the House of Delegates vote, Mayor Rahm Emanuel urged the CTU to call off the one-day strike and remain where they and CPS students belong: in the classroom.

Contributing: Fran Spielman