SEATTLE (AP) — Washington voters will decide whether to possibly make the state the first in the nation to legalize recreational pot use, setting up a potential showdown with a federal government that backs the drug's prohibition.

Initiative 502 would establish a system of state-licensed marijuana growers, processors and retail stores, where adults over 21 can buy up to an ounce. It also establishes a standard blood test limit for driving under the influence.

Estimates have shown pot taxes could bring in hundreds of millions of dollars a year, but the sales wouldn't start until state officials make rules to govern the legal weed industry.

Recent public polling has showed significant support for the measure. Pot legalization initiatives were also on the ballot Tuesday in Colorado and Oregon.

Promoted by New Approach Washington, I-502 calls for a 25 percent excise tax at each stage from the growers on until it is sold in stores to adults 21 and over. They could buy up to an ounce of dried marijuana; one pound of marijuana-infused product in solid form, such as brownies; or 72 ounces of marijuana-infused liquids.

The cannabis would be subject to testing to establish its THC content, and labeled accordingly. Public display or use of marijuana would still be prohibited.

State financial experts estimate it could raise nearly $2 billion in tax revenue over the next five years, with the money going toward education, health care, substance abuse prevention and basic government services.

It remained unclear how the federal government would respond.

When state and federal laws conflict, federal law takes precedence. Federal authorities could sue in an attempt to block I-502 from taking effect. The Justice Department has given no hints about its plans.

The campaign was notable for its sponsors and supporters, who ranged from public health experts to two of the DOJ's top former officials in Seattle, U.S. Attorneys John McKay and Kate Pflaumer.

The effort raised more than $6 million in contributions, with more than $2 million of that coming from Progressive Insurance Co. founder Peter Lewis, who used marijuana to treat pain from a leg amputation.

The ample fundraising allowed New Approach Washington to run television ads through the campaign's final weeks.

Meanwhile, I-502 had little organized opposition. Some in law enforcement and public health are concerned that increased access will lead to increased abuse, especially among teens.

Others who opposed the measure did so because it didn't go far enough, and that the blood test limits were arbitrary and could affect medical marijuana patients. Still others worried about a possible federal-state law clash.