With 61,680 votes to spare out of just over 1.934 million cast, Missourians last November passed the Proposition B "puppy mill" law by a 51.6 percent margin.
On Thursday morning, with six votes to spare out of 34 cast, the Missouri Senate approved and sent to the House a major rewrite of that law.
"The Senate decided to defy the will of the voters and to dismantle Proposition B provision by provision, dealing a blow to the welfare of dogs and also to the democratic process in Missouri," Barbara Schmitz, the Missouri state director for The Humane Society of the United States, said in a news release.
The HSUS was a major sponsor of Proposition B, helping write the law and circulate petitions to get the issue on the Nov. 2 ballot.
But several senators said during the debate and after the vote that Proposition B had presented some legal problems.