The 1.4 percent motor vehicle excise was approved in 2002 by voters. A King County Superior Court judge, William Downing, ruled last April that it is constitutional.
But opponents say the tax is arbitrary because it is based on a state table that values cars higher than the Kelley Blue Book value.
The Seattle mayor and city council have dropped their support for the monorail project. It's going back to voters for a fifth vote in November on a shortened line.
As it stands now, even if the monorail project is scrapped, the tax would have to be collected for another 2 1/2 years to pay off the debt the project has already accumulated.