The alliance has released an email sent to a mailing list by Montana Quarter Horse Association president Stan Weaver.
Alliance representatives John Holland and Vicki Tobin said, in their view, Weaver appeared to be rejoicing that Montana may be home to a horse-killing plant.
The bill, sponsored by Representative Ed Butcher, has proved controversial on several levels.
Montana Governor Schweitzer returned the bill to the state legislature over concerns it limited citizen's rights through courts to challenge construction of a plant.
However, both houses returned the bill unchanged. Schweitzer's decision not to sign the bill meant it passed automatically into law.
The alliance says the new law has left many Montana legislators and citizens shocked that their state might soon be known as the new "home of horse slaughter".
"Montana has enacted a probably unconstitutional statute that denies due process under the United States Constitution," Holland and Tobin said in a statement.
Horse slaughter, they said, will tarnish the "Big Sky" brand and everything it stands for, from cattle to tourism.
"History has shown that such plants bring nothing but pollution and controversy. Montana law makers failed to ask themselves why Texas and Illinois, and now Saskatchewan Canada, have rid themselves of the industry."
Weaver's email praises members for pushing the legislation while discussing the haste with which it was put together.
Weaver describes how the MQHA and Ed Butcher had come up with the idea for the bill just weeks before it was introduced.
Weaver wrote: "I also would like to thank the Montana Quarter Horse Association and its Board of Directors for letting me represent them in this endeavour. It was at a meeting of these folks on Sunday, February 8th that the whole idea was born. At that meeting it was decided unanimously that we would support the Horse Processing bill, and I agreed to write a letter [to be sent] to our members. The effort and email list just grew from there."
"This magnificent breed," said Holland and Tobin, "touted as the most versatile of all horses, is being sent to slaughter in record numbers. In fact, half of all horses sent to slaughter each year are American Quarter Horses.
"Weaver is apparently so enamored at the prospect of a slaughter plant to butcher Montana's Quarter Horses that he ponders writing a book that will contain all the emails and letters in support of horse killing."
The pair accused the American Quarter Horse Association (AQHA) of continuing to promote indiscriminate breeding.
"Last year, when other businesses were reducing production, AQHA management and its member breeders continued their mad quest to grow revenues by registering 140,000 new foals, an increase of 5000 more horses over 2007," they said.
In his speech before the 2008 annual convention, Bill Brewer, the AQHA’s then executive vice-president said, "Our challenge becomes looking at ways to introduce an equine economic stimulus package that will boost registration numbers."
Weaver’s ranch alone produces and registers 100 horses a year "and helps fill the AQHA treasury with registration fees", the alliance says.
It argues that vested interests promote stories that the United States is being flooded with tens of thousands of abandoned horses.
It was picked up by the Montana media and repeated without question, they said, even though county officials reported a total of only 14 abandoned horses in 2008. Yet little or nothing was reported on the breeding encouraged by the AQHA.
"According to Weaver, the next major AQHA effort will be to try to defeat the federal legislation that will end the slaughter of American horses; HR 503, The Prevention of Equine Cruelty Act of 2009.
The pair said they expected more of the "elaborate disinformation campaign" from pro-slaughter lobbyists.