The Arizona Supreme Court is set to hear a challenge from Republican state lawmakers to the Medicaid expansion pushed through the Legislature four years ago by then-Gov. Jan Brewer.
The state's highest court will hear arguments on Thursday.
The state court of appeals ruled against the lawmakers, saying a hospital assessment that pays the state costs of insuring more than 400,000 low-income residents is constitutional.
Republican lawmakers who sued call the assessment a tax requiring a two-thirds majority vote to pass.
Brewer battled fellow Republicans in the Legislature for months in 2013 over the expansion.
The Goldwater Institute is representing the GOP lawmakers.