Texas is one of several states that have refused to accept the expansion of Medicaid under the 2010 healthcare law. The Patient Protection and Affordable Care Act (PPACA) required states to expand Medicaid, the federal and state-subsidized health program for lower income Americans, to people with incomes up to 138 percent of the federal poverty level.

Last year's Supreme Court decision took away this requirement and gave states the option of participating in Medicaid expansion. States that agree to expand the program will get federal aid to help them do it.

Castro pointed out that the expansion is "already 100 percent paid for," and that refusing it would allow the money to be spent in other states. At the same time, preventing the expansion of Medicaid would deny families needed access to healthcare.

"While most states have begun to take the steps necessary to increase health care access, the governor's stubbornness has continued to keep Texas on the embarrassing five-year track as the state with the highest rate of uninsured individuals and children," he wrote.

Perry, who ran for president last year but dropped out in January shortly after placing fifth in the Iowa caucus, told the Obama administration last summer that he would not participate in Medicaid expansion in his state.

"Medicaid is a system of inflexible mandates, one-size-fits-all requirements, and wasteful, bureaucratic inefficiencies," he wrote in July. "Expanding it as the PPACA provides would only exacerbate the failure of the current system, and would threaten even Texas with financial ruin."

Republicans have pointed out that while the healthcare law would pay for Medicaid expansion at the state level in the short-term, this aid would stop after a few years and stick states with the full cost of the expansion.

House Republicans have proposed legislation that would repeal the law's language on Medicaid expansion. The sponsor of that bill, Rep. Matt SalmonMatthew (Matt) James SalmonArizona GOP tinkers with election rules with an eye on McCain's seat Quiet jockeying for McCain seat angers Republicans McSally tells GOP colleagues she'll run for Arizona Senate MORE (R-Ariz.), says the bill "essentially bribed states" to accept the expansion.