Amid looming deficits and ballooning overtime bills, the MTA wants to change workers’ contracts to cut back on their vacation days and overtime, new documents show.

Transport Workers Union Local 100, its largest union, posted the transit agency’s Aug. 14 offer to its members online Monday — as it unanimously rejected the proposals, which it labeled “insulting.”

Faced with massive anticipated future deficits of up to $1 billion and overtime costs that totaled $1.3 billion in 2018, the MTA has sought major concessions from TWU and other unions representing its workforce as they renegotiate lapsed contracts.

The agency’s latest offer targets specific contract rules that agency officials blame for growing overtime expenses.

Agency officials want to limit overtime earnings to employees who’ve worked over 40 hours in a single week, as opposed to more than eight hours in a single day.

They also want more leeway to hire part-time bus drivers for peak hours and outside contractors for responsibilities such as station and car cleaning.

The offer also includes strategies to reduce the number of workdays missed by bus and subway workers — another driver of overtime costs. Bus and subway workers missed an average of 54 days of work in 2018, The Post revealed last week.

One provision would require the workforce to reduce that average by three days by the end of 2020 — or else give management the power to implement its own work-rule changes to achieve that goal. Employees would not be permitted to bank earned overtime as time off until that goal is achieved.

Another provision would cut the number of vacation days for new employees. The average bus and subway worker took 18 vacation days last year.

Management also wants to double employee health care contributions — from 2 percent currently to 4 percent by Jan. 1, 2023.

In flyers distributed to rank-and-file workers after rejecting the offer, TWU officials accused MTA chairman Pat Foye of “hiding behind the false narrative of budgetary woes.”

“Transit workers have worked too hard to improve service — and this union has worked too hard over decades to establish a decent standard of living for our members — to now go backwards,” TWU Local 100 president Tony Utano said in a statement.

“We will do everything in our power to fight these insulting contract demands.”