Colorado voters have approved a measure making it tougher to change the state's constitution.

Amendment 71 requires those seeking to place a constitutional amendment on the ballot to get voter signatures in all 35 state Senate districts. It also means 55 percent of voters will have to approve future amendments.

Currently, a simple majority can approve amendments, and petition signatures can be gathered anywhere.

Supporters say too many citizen petitions have produced conflicting constitutional mandates on state spending. They prefer initiatives that change state law and can be modified as needed.

Opponents said only well-funded special interest or industry groups could afford to petition - or fight a petition - under the amendment.