It would be the most sweeping expansion of school choice in Arizona since charter schools opened their doors two decades ago.

Republican State Sen. Debbie Lesko's SB 1431 would let every parent in Arizona use state tax dollars to send their children to private or religious schools.

Backers of school choice, such as Education Secretary Betsy DeVos and Gov. Doug Ducey, call that "letting the money follow the child."

RELATED: Lesko's appearance on Sunday Square-Off

If a parent wants a child to go to religious school, they say, the parent should be able to use tax dollars to pay for it.

But despite supporters' claims that universal school choice saves taxpayer dollars, 12 News' reporting shows the state would have to spend more tax dollars on children who attend a private school after ditching a public school.

The short-term hit to the state's general fund could grow by more than $5 million a year for the next few years.

"I'm afraid for what this will do for the kids of Arizona," said Wendy Garcia, a mother of two public school students who organized a protest Monday outside Lesko's Peoria home.

"I'm watching her," Garcia said of Lesko and her bill.

So-called ESAs -- empowerment scholarship accounts -- are already being used by about 4,000 schoolchildren in specific demographic groups.

Parents get debit cards worth about $5,200 a year -- the cost of one year's tuition at many religious schools.

Lesko's bill would phase in universal vouchers by the 2020-21 school year.

In a recent interview on "Sunday Square Off," Lesko told me vouchers would deliver a huge tax saving to Arizonans.

"They give parents another option to get the best educational choice for their child and they also save taxpayer money -- about 4,300 per year per student," she said.

Saving money is a big selling point for conservative Republicans touting ESAs.

Here's Lesko's math: Our schools get about $9,500 per pupil in local, state, and federal money. The vouchers cost the state about $5,200 per child. So she claims a $4,300 saving in total tax dollars that go to schools.

Lesko's math is misleading, but that doesn't stop it from reappearing every time ESA legislation advances. The problem is her bill affects only state tax dollars -- it doesn't directly impact federal or local money.

Here's the bottom line on what an ESA expansion would cost:

Our public schools get about $4,200 per pupil in state aid. Vouchers cost the state $5,200 per child. So ESAs could cost the state general fund an additional $1,000 for every child who leaves a public school for a private or religious school.

Charter schools are treated differently from public schools. The state would save money -- about $600 per child -- on charter school students who leave for private schools.

But the evidence so far is that most ESA students come from high-performing schools in wealthy public school districts. Public school students represent more than eight of every 10 K-12 students in Arizona.

Could local property taxes and federal spending for public schools decline in the long term?

The biggest unknown for property taxes is whether there's a large migration to private schools. DeVos, not ESAs, might be the main threat to federal spending on schools.

Supporters argue there's a cap on new ESAs at 5,500 a year. That cap means with Lesko's expansion, Arizona could have 9,500 ESA students a year from now, 15,000 the year after, and 20,000 the year after that.

Each successive expansion could cost the general fund up to $5.5 million.