Idaho Supreme Court justices expressed skepticism Jan. 29 about a challenge to the state’s planned expansion of Medicaid to cover people who make a little above the federal poverty line.

Voters approved a ballot initiative in November by 61 percent to expand eligibility for the low-income health program. The libertarian-leaning Idaho Freedom Foundation says the expansion, slated to go into effect in 2020, is an unconstitutional delegation of state legislative authority to the federal government.

By approving the expansion, voters essentially vetoed their overwhelmingly Republican Legislature. State lawmakers have for years declined to expand the program to cover tens of...