KEVIN MCCOY

USA TODAY

The lowest-paid hourly workers in four states won a boost in their minimum wage to at least $12 an hour in Tuesday's voting outcomes.

Voters in Arizona, Colorado, Maine and Washington approved increases in their states' respective hourly pay rates to at least $12 by 2020, according to late election results compiled by The Associated Press and Ballotpedia, a nonpartisan online encyclopedia of elections and politics.

Roughly 2 million workers in the four states will see their wages increase, according to the National Employment Law Project, a worker advocacy organization.

Separately, South Dakota voters rejected a law that would have reduced the state's $8.55-per-hour minimum wage to $7.50 for non-tipped employees under age 18.

The results further expand the pay gap with the 21 states where workers are covered by the $7.50-an-hour federal minimum wage, U.S. Department of Labor data shows. Those states collectively account for less than half the nation's workforce.

In Arizona, the outcome means the state's current $8.05-an-hour minimum wage will rise to $10 next year and gradually increase to $12 in 2020. Starting in 2021, the pay rate will be adjusted annually based on the state's cost of living.

Colorado voters similarly approved a gradual hike in the state's $8.31-an-hour minimum wage, starting with an increase to $9.30 per hour next year. The pay rate will increase 90 cents in successive years until it reaches $12 per hour in 2020. The rate will also get an annual cost of living adjustment as of 2021.

Maine's current $7.50-an-hour rate will also rise to $12 by 2020. The change will start with an increase to $9 per hour in 2017, followed by three years of $1-an-hour hikes. Increases after that would be determined by inflation.

Washington voters approved an even larger increase. The western state's current $9.47-an-hour minimum wage will rise to $13.50 by 2020, starting with a jump to $11 next year. Cost of living adjustments would determine the rate after 2020.

The approvals are the latest in a national movement to boost incomes for the nation's lowest-paid employees. In recent years, approximately 20 states and dozens of cities and counties have approved minimum wage hikes via legislation or voter referendums while Congress has kept the federal pay floor unchanged.

The latest increases "will bring very badly needed pay hikes for 2.3 million workers," said Paul Sonn, general counsel for the National Employment Law Project. He called minimum wage guarantees "one of the most popular public policies in America."

The outcomes in Arizona, Colorado, Maine and Washington resulted in part from funding gaps that saw labor unions outspend business organizations on voter education and outreach on the ballot issues, said Michael Saltsman, research director for the Employment Policies Institute, a business-backed think tank focused on labor policy.

"It's going to be difficult for businesses in those four states to adapt to the increases," said Saltsman.