If 19 more states expand Medicaid, an additional 4 million people would have healthcare coverage, according to a high-ranking Obamacare official who called for holdouts to expand the entitlement program.

"We want all states to extend important coverage," said Health and Human Services Secretary Sylvia Burwell Wednesday during an event commemorating the 50th anniversary of Medicare and Medicaid.

Obamacare expanded the low-income healthcare program, but a Supreme Court ruling in 2012 said states had to agree to the expansion.

So far, 31 states and the District of Columbia states have expanded the program. That has resulted in about 23 million people getting coverage, Burwell said.

The administration is open to working with states to find "approaches that work for them" to expand Medicaid, she said.

Several red states have expanded the program in exchange for a waiver from the administration for certain provisions. The most recent red state to employ that method was Montana, which applied for a waiver to ensure new beneficiaries pay a small part of their coverage.