Missouri voters blocked a Republican-led effort to raise the bar for passing citizen-led constitutional amendments, NBC News projects.

Under current state law, such ballot measures require a simple statewide majority for passage, a process that has been used in recent years to grant abortion rights, expand Medicaid and legalize marijuana.

But Amendment 4 would have required citizen-led ballot initiatives to win a majority in each of Missouri’s congressional districts, boundaries that are drawn by state lawmakers. Other ballot measures, like those referred from the Legislature, would not have been subject to this higher bar.

It would have created “the strictest passage threshold in the country for citizen-initiated ballot measures only,” according to Hannah Ledford, the deputy director of the Fairness Project, a group that supports citizen initiative processes. She added it could have left Missouri’s direct democracy process “functionally dead.”

Missouri is one of 26 states that allows voters to legislate directly by petitioning to place a constitutional amendment on the ballot.

The amendment was first proposed by Republican state Rep. Ed Lewis, after Missouri voters used the method to advance a spate of progressive priorities in a state where the GOP has full control of government. Many recent amendments would have failed under the stricter standard proposed by Amendment 4.

“This amendment is really an amendment to protect our Constitution,” Lewis said on a radio program earlier this month. “We have the absolute lowest and weakest process and that’s why we’ve had six initiative petitions passed in the last eight years that have really done real harm to our state and to our Constitution.”

The state’s Republican-controlled Legislature has repealed or limited some changes made through ballot initiatives, including repealing paid sick leave.

Opponents of the proposal heavily outspent its supporters, pouring more than $6 million into broadcast ads, according to data from the tracking firm AdImpact. The measure’s backers did not air any broadcast ads, AdImpact’s data shows.

The more-moneyed campaign against Amendment 5, a proposal to end income taxes and expand sales taxes, also included arguments against Amendment 4 in their ads.

The effort to change the constitutional amendment process also came after Republican lawmakers redrew the state’s congressional map last year to carve up the lone Democratic-held district. Earlier Tuesday, Missouri Secretary of State Denny Hoskins said that a referendum that sought to undo the new map would not appear on the state’s ballot, arguing the effort was unconstitutional.

Missouri isn’t the only state where Republicans have sought to make it more difficult for citizen-led constitutional amendments to pass. In November, North Dakota and South Dakota voters will decide whether to bump the threshold up from a statewide majority to 60%. And in Utah, there will be a similar measure on the fall ballot specifically related to tax increases.