The Ohio Supreme Court struck down a congressional map skewed to favor Republicans on Friday, ruling it was the equivalent of a dealer stacking the deck and sending it back to state lawmakers to try again.

The map would have given Republicans a 12-to-3 advantage in elections for the House of Representatives, even though the G.O.P. has recently won only about 55 percent of the statewide popular vote.

Under an amendment to the Ohio Constitution overwhelmingly passed by voters in 2018, mapmakers are not allowed to redistrict the state in a way that unduly favors one party. The proposed map was drawn by Republicans in the State Legislature and passed without Democratic support, and the court rejected it in a 4-to-3 decision.

“When the dealer stacks the deck in advance, the house usually wins,” wrote Justice Michael Donnelly for the majority, adding that the Republicans’ plan was “infused with undue partisan bias.”

“This is not what Ohio voters wanted or expected,’’ the court ruled.