A plan to provide a little financial relief for Sound Transit car tab payers was in peril Monday.

Most lawmakers in the House and Senate want to change how the motor vehicle excise tax is calculated and save owners of roughly 2.5 million vehicles a collective $780 million in payments to the regional transit authority in the next few years.

Majority Democrats in the Senate also think it’s critical to offset the loss of revenue to Sound Transit to keep its voter-approved expansion plans on track. But their House counterparts disagree and think the transit agency’s concerns don’t need to be resolved before the session’s scheduled end Thursday.

“I don’t think we have to decide by Thursday,” said Rep. Judy Clibborn, D-Mercer Island, chairwoman of the House Transportation Committee. House Democrats are focused on reducing the car license fee with a fairer valuation system and “we can deal with (Sound Transit) in the future when we know what the impacts are.

“If they want a bill to help provide fairness on car tabs, they have the means to do it,” she said, referring to the Senate. “It’s in their court.”

Sen. Steve Hobbs, D-Lake Stevens, chairman of the Senate Transportation Committee, responded by saying if the state provides no offsetting aid, it is likely it will delay or derail projects such as extending light rail service to Everett and Tacoma.

“If you’re going to create a $780 million hole, all you will be doing is jeopardizing projects in Snohomish and Pierce County,” he said. “I don’t know how I can vote for something that takes away projects from Snohomish County.”

Sound Transit 3 passed in November 2016 on the strength of support in Snohomish and King counties. It was rejected by voters in Pierce County.

The plan calls for adding 62 miles of new Link light-rail line, including an extension to Everett Station by 2036, via the Paine Field industrial area. Other new light-rail destinations envisioned in ST3 include Tacoma, Ballard, West Seattle, downtown Redmond, south Kirkland and Issaquah.

To pay for the upgrades, the sales tax went up half a percent within the district. There’s also a new property tax assessment of 25 cents for each $1,000 of assessed valuation. And the tax rate for figuring car tab fees went from 0.3 percent to 1.1 percent, resulting in a near trebling of costs for some vehicle owners.

As part of the package, lawmakers created the Puget Sound Taxpayer Accountability Account to ensure some of Sound Transit’s future sales tax payments on projects are spent in the transit district only on serving educational needs of young people who are low-income, homeless or in foster care.

The account is empty now. But over the next quarter century, it is expected to fill with $318 million for King County, $111 million for Pierce County and $89 million for Snohomish County.

The Senate passed a bill letting Sound Transit decide to use that money for projects if needed. If not, it would be spent as originally envisioned.

“We have to give them the flexibility,” Hobbs said. “We crafted a bill that cuts the car tabs and pays for projects.”

But on Saturday, the House Transportation Committee stripped that provision out of the bill then passed it. The slimmed-down version awaits action on the House floor. It is pretty much the same content as a car-tab reduction measure the House passed in January but never got a hearing in the Senate.

Rep. Lillian Ortiz-Self, D-Mukilteo, who serves on the transportation panel, voted to rewrite the Senate bill.

“I want to see the projects done on time. I want to give consumers a reduction in their car tabs,” she said. “And I’d like to see the money back-filled but without using those education dollars because we would be taking it away from services for our must vulnerable students. There should be a way to do all three.

A solution can be found before session ends if the Senate doesn’t dig in too deep, she said.

“There are other ways to fund it,” she said. “I’m hoping they’ll be open to more options.”

A version of this story first ran in the Everett Herald. Jerry Cornfield can be reached at 360-352-8623 or jcornfield@heraldnet.com. Follow him on Twitter at @dospueblos.