The Equine Welfare Alliance claims the actions of the seven senators "may have ended once promising careers".
Senators Terry Murphy, Taylor Brown, Ken Hansen, Verdell Jackson, Rick Ripley, Bradley Hamlett and Donald Steinbeisser should be removed from office, the alliance said.
Alliance representatives John Holland and Vicki Tobin, in a joint statement, said the group promised to mobilise thousands of volunteers to assure their defeat.
"Tens of thousands of humanitarians across Montana are opposed to horse slaughter. Survey after survey has shown that more than 70 percent of all Americans oppose the practice.
"A poll on a respected on-line paper in Montana on the day of the vote, reflected 76% of readers against building a plant.
"Our volunteers are motivated, computer savvy and willing to spend money to support the candidates of their choice. Moreover, some national political action committees share our belief that horses should not suffer.
"At a time when Americans are finding themselves struggling to make ends meet and joining the ranks of the unemployed, we question the wisdom of wasting taxpayer dollars on the legislative efforts and building a plant that cannot operate legally in America for a cruel business that serves the owners of less than 2% of the horse population.
"We question the wisdom of bringing yet another foreign business back to our country that does not offer any economic stimulus to the state of Montana or their citizens.
"The only sector to benefit from such a business will be the lawyers in Montana. Reserve your court dockets now. Montana will find themselves in litigation for years trying to collect pollution, abuse and cruelty violations."
The alliance called for the defeat of every politician who votes to bring horse slaughter back to US shores. "We are motivated, we can raise millions in small donations and we will use it," the group said.