Steve Yarbrough is one of the most powerful men in Arizona. As president of the State Senate, he has promoted a range of conservative policies, including a tuition tax credit system that provides over $100 million per year to finance vouchers for private schools.

In his speech to Congress this week, President Trump singled out a young woman who attended private school using a tax credit-financed voucher. The president urged Congress to pass legislation that would provide similar benefits to millions of students.

But Mr. Yarbrough is not just a champion of tax credit vouchers. He also profits from them personally. The story of how that happened raises questions about President Trump’s campaign promise to spend $20 billion to increase school choice. There’s a strong chance that he’ll do that through tax credit vouchers — a mechanism that Betsy DeVos actively campaigned for before she became Mr. Trump’s education secretary.

State tax credit voucher programs have grown rapidly in recent years. The number of students receiving them increased to 256,000 this year, from about 50,000 in 2005. Arizona has one of the oldest and largest programs. It allows taxpayers who donate money to nonprofit voucher-granting organizations to claim a 100 percent, dollar-for-dollar credit against their state taxes (up to a certain limit). In other words, if a married couple donates $1,000 to a voucher-granting nonprofit, their tax bill is reduced by $1,000. The nonprofit then gives the money to families who use it to pay tuition at private schools.