There's a simple reason teachers across the country are walking off the job: in many states, their paychecks are getting smaller.

The latest protests brought a wave of red-clad teachers to the Arizona state Capitol Thursday for a walkout that closed most of the state's public schools as teachers in neighboring Colorado staged a similar rally in Denver.

Thousands of teachers were expected to march through Phoenix to demand a 20 percent raise for teachers, about $1 billion to return school funding to pre-Great Recession levels and raised pay for support staff, among other things.

The walkouts are part of a widening protest that began earlier this year with a grass-roots movement known as #RedforEd that sparked teacher walkouts in West Virginia, Oklahoma and Kentucky.

Across much of the country, a decade-long economic recovery has created millions of new jobs and begun to nudge average wages higher. While the median household income stagnated in the years immediately following the Great Recession, paychecks have begun rising again for most Americans.

But in more than half of the states, teacher salaries have fallen, in some cases sharply. As of the 2016-2017 school year, the latest data available, the average inflation-adjusted salary for elementary and secondary school teachers was lower than it was nearly two decades earlier.

Teachers in Colorado and Arizona have been among the hardest hit, according to data from the National Center for Education Statistics.