Amendment 71 requires petition signatures from at least 2 percent of registered voters in each state Senate district and a current total of 98,000 valid signatures statewide.
The lawsuit claims getting signatures in rural districts dilutes the signatures from heavily populated areas.
Voters can amend their state's constitution: Think legalization of pot. But some Coloradans say citizen initiatives are out of control, and a well-funded push is underway to raise the bar.
On the ballot 2016: Amendment 71 would make it harder for citizen-backed state constitutional amendments to get on Election Day ballots.