The United Auto Workers union is readying plans to strike against Detroit's Big Three automakers after rejecting Ford, General Motors and Stellantis' latest contract offers with only hours remaining for negotiations before Thursday's midnight deadline.

While there is still time for the automakers to reach tentative agreements with the union before any work stoppage, the parties were still far apart on terms as of Wednesday night, and each is preparing for plant shutdowns.

The union was seeking 40% pay raises over four years along with an array of additional benefits, including a reduction of the workweek to 32 hours for 40 hours worth of pay for its 146,000 members at Ford, GM and Dodge parent Stellantis.

UAW President Shawn Fain told members Wednesday night in a Facebook live video that Ford has offered autoworkers 20% raises over the term of the contract, while GM and Stellantis proposed 18% and 17.5%, respectively, all of which the union rejected as inadequate.

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"We're making progress ... but we're still very far apart on our key priorities," Fain said. "To win, we're likely going to have to take action." 

When asked for a status update Thursday afternoon, a Ford spokesperson told FOX Business, "We want to negotiate, but we have not received a counteroffer yet."

GM said it "put another economic offer on the table this morning with the goal of avoiding a work disruption by 11:59 p.m. tonight," and Stellantis did not immediately respond to a request for comment Thursday.

Fain has warned repeatedly that the UAW will strike against any automaker that it has not reached an agreement with by the deadline. If the union strikes at Ford, GM and Stellantis, it would be the first time the union launches a strike against all three simultaneously.

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While the UAW is poised to strike against all the Big Three, it plans to use an unconventional approach. Rather than launching an across-the-board strike, the union plans to strike only at certain plants initially, then incrementally strike at additional factories to ramp up pressure if negotiations drag on for days past the current contract expiration.

An extended work stoppage would be costly not only for the auto industry but for the economy at large. 

If the UAW does end up striking against all three automakers at once and manufacturing operations are entirely shut down, it would cost the U.S. economy roughly $5.6 billion in just 10 days, according to an analysis from Anderson Economic Group. 

Any automaker whose production is halted will suffer earnings losses of roughly $400 million to $500 million per week, Deutsch Bank has estimated.

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The UAW is planning a rally in Detroit Friday featuring Fain, Sen. Bernie Sanders, I-Vt., and other lawmakers to coincide with the anticipated first day of walkouts.

Reuters contributed to this report.