Unlike nearby Kansas and Nebraska, Oklahoma fell short of enacting a serious income tax cut last year. Republican Gov. Mary Fallin was elected in 2010 pledging to phase out the income tax over several years, starting with a rate cut in 2013. But even though Republicans control the state house and hold nearly two-thirds majorities in both houses of the legislature, the tax cut got tripped up by GOP members who wanted to spend more on district pet projects rather than cut tax rates.

In an interview, Ms. Fallin tells me that...