Colorado voters on Tuesday rejected an effort to dramatically change how the state’s elections are conducted.

As of 10 p.m., 55.56% of Colorado voters opposed changing the electoral procedures for the state legislature and state offices — governor, attorney general, treasurer, secretary of state, the State Board of Education and the University of Colorado Board of Regents — as well as federal races for Colorado’s seats in the U.S. House of Representatives and the U.S. Senate.

Just over 44% of voters supported Proposition 131. More than 2.2 million votes had been counted so far.

“This is a win for Colorado voters, and a win for democracy against a historic spending spree by wealthy individuals and special interests on a Colorado ballot measure,” Sean Hinga, a spokesperson for the anti-Proposition 131 campaign, said in a statement. “Let this be a lesson to big money that grassroots power is still alive and well in Colorado when voters do their homework and cast a ballot.”

The measure would have replaced the state’s current primary system with a single open primary in each affected race — meaning all candidates would compete against each other in a primary, regardless of party.

Proposition 131 would also have instituted a ranked-choice voting system, under which voters in the general election would rank each candidate in a given race by preference.

The top four vote-getters in the primary — regardless of party — would advance to the general election. If no candidate earns a majority in the first round of voting, the candidate with the fewest top rankings gets eliminated. Any ballot that had that candidate as the top choice then automatically shifts to that voter’s next-highest-ranking candidate, and the tabulation begins again. It continues in rounds until one candidate secures a majority of votes.

Proposition 131 “would have sacrificed the safety and security of our election system for the whims of special interests and big corporations whose pay-to-play tactics would have flooded the state with even more dark money,” Shad Murib, chair of the Colorado Democratic Party, said in a statement.

The measure was the brainchild of Denver millionaire Kent Thiry, who has successfully placed other election-related measures on the ballot in recent years.

“Reforms of this magnitude take time and effort,” Thiry said in a statement conceding defeat Tuesday evening. “Campaigns that have been on the right side of history — from women’s suffrage, to civil rights, to marriage equality — were all journeys that experienced defeat before finding overwhelming victories. This is just one step on our journey for open primaries and ranked voting.”

The campaign was among the most expensive Colorado ballot measure efforts of this century, according to state campaign finance data. As of Monday morning, the Colorado Voters First campaign had raised more than $18.1 million (a figure that includes $135,000 in non-monetary support) to back Prop 131.