SALT LAKE CITY -- A medical marijuana advocacy group has threatened a lawsuit over involvement The Church of Jesus Christ of Latter-day Saints has had with members of the Utah State Legislature in crafting a "compromise" bill to replace Proposition 2.

FOX 13 has obtained a "preservation letter" sent by Together for Responsible Use and Cannabis Education (TRUCE) attorney Rocky Anderson to LDS Church lobbyists, the Utah Patients Coalition, the Utah Medical Association and members of the Utah State Legislature. It calls on them to keep records in the event of a lawsuit.

"We were contacted by these people and organizations for the first time during the past two days. We are investigating a legal challenge to (1) the calling of a special session of the Utah Legislature at the behest of The Church of Jesus Christ; (2) any effort, in collusion with or at the behest of The Church of Jesus Christ, to materially alter the initiative statute supported by a majority of voters who passed Proposition 2 in the recent election; and (3) the long-term pattern of domination of the Utah Legislature and the interference in the functions of Utah government by The Church of Jesus Christ," the letter states.

Anderson said he has been retained by TRUCE and its founder, Christine Stenquist, and the Epilepsy Association of Utah and its incoming president Doug Rice (who also is a member of TRUCE).

"We literally are David in this David-Goliath story," Stenquist said Thursday. "And this is my little pebble and I’m loading up in my slingshot."

On Wednesday, Rice told FOX 13 the group was contemplating a lawsuit if lawmakers moved ahead in a Dec. 3 special session and swapped out Prop. 2 with the "compromise bill" that has been the subject of closed door talks between initiative sponsors Utah Patients Coalition, Libertas Institute, and opponents including the LDS Church and Drug Safe Utah.

Connor Boyack, the president of the Libertas Institute, defended the compromise bill and said it was 90% of what was in Prop. 2. He insisted the compromise averted the legislature potentially gutting the voter-approved initiative.

But TRUCE has demanded the Utah State Legislature implement Prop. 2 only, that voters approved. The group, which was not on Utah's Capitol Hill as negotiations were under way on Wednesday, has urged its supporters to contact lawmakers and Governor Gary Herbert to demand they follow the will of voters.

Prop. 2 has passed with about 53% of the vote.

"As members of the community, we have worked, from the outset, with medical professionals, law enforcement, educators and many other groups and prominent community leaders to seek the best for the people of Utah, to provide relief from human pain and suffering, especially where children are concerned," LDS Church spokesman Doug Andersen said in a statement. "Broad community engagement was the reason a workable, beneficial and safer medical cannabis program was put together at the direction of state leadership. We stand behind and look forward to the safe, responsible and compassionate solution that will be considered by the state legislature."

The LDS Church has faced heavy criticism over its involvement in Prop. 2. The faith vocally opposed the initiative and urged its followers to vote against it. It also engaged in the negotiations over the compromise bill as a "community stakeholder."

Anderson, who is a former mayor of Salt Lake City, offered pointed criticism of the LDS Church's outsized involvement in Utah politics in his letter.

"The Church of Jesus Christ has caused the sponsorship, passage, and/or defeat of many public laws—state, county, and municipal—for many years. It is common knowledge that no liquor bill, sex education bill, gambling bill, or sexual orientation/gender identity bill will be passed by the Legislature without the support of The Church of Jesus Christ," he wrote.

In an interview with FOX 13, Anderson said he experienced it when he was mayor.

"There was no question the LDS church told members of the council how to vote," he said.

Asked if the Church told him how to vote, he chuckled and replied: "Nah, they knew better than that."

House Speaker Greg Hughes, R-Draper, issued a scathing retort to the letter in a text message to FOX 13.

"I think Rocky is gracious to include me in his latest Kabuki Theater act. I see that former Senate President Waddoups also has a supporting role. As I am retiring from the House I’ll miss serious public service but at least I’ll have time to play along!" he wrote.

Speaking to reporters at his monthly news conference on KUED, Governor Gary Herbert said churches have a right to voice their opinions on certain issues.

"I think certainly the LDS Church has influence because most of the people of Utah, the majority, happen to be members of that church and just by extension, there’s probably influence," he said. "But you could say the same thing about Massachusetts, there’s the Catholic Church."

The governor noted that supporters and opponents came together to craft the compromise and he gave his support to the special session replacement bill.

"We’re in a good place here and I’m looking forward to the special session," he said.

Religions can weigh in on social issues in politics without fear of losing their tax exempt status. However, they cannot weigh in on political candidates themselves. The Islamic Society of Greater Salt Lake and the Episcopal Diocese of Utah joined the LDS Church in opposing Prop. 2.

The Catholic Diocese of Salt Lake stayed out of the medical marijuana ballot initiative, but urged its followers to actively work to pass Proposition 3, Medicaid expansion.

Stenquist said she knows faiths can speak out on issues.

"Absolutely they can. But we have an egregious amount of weighing in in this state," she said of the LDS Church. "And there is a clause in the constitution that definitely says there needs to be a separation of church and state."