Some of these 18- to 24-year-olds, Karger said, will be featured in a series of TV commercials to be aired in Utah early next year, encouraging others to step forward with inside information on the church's business holdings and political dealings including its efforts against same-sex marriage.
"Somebody has got to fight for these kids," Karger said. "It's inexcusable, the damage and suffering the church has caused for so many of these families."
The Mormon faith, he said, "needs to change with the times."
Contacted on Thursday, a public-affairs spokesman for The Church of Jesus Christ of Latter-day Saints declined to comment.
Sam Brunson, a tax-law scholar at Loyola University in Chicago, has noted that no tax-exempt entity has lost its exemption with the IRS for violating public policy since 1978 "much less," he said, "any church."
"Which is to say, the [Mormon] church will not lose its tax-exempt status," Brunson wrote in a July 2015 blog for the By Common Consent website soon after gay marriage became legal in all 50 states.
Karger, who in 2012 became the first openly gay candidate to seek the Republican presidential nomination, founded Californians Against Hate in 2008 partly to investigate underreported financial involvement by Mormons in support of Prop 8, which briefly banned gay marriage in the Golden State.
Karger said Thursday his probing of the LDS Church had fallen largely dormant after his failed presidential quest only to be revived in the aftermath of the November 2015 LDS policy declaring gay Mormon couples "apostates" and generally barring their children from baptism until they turn 18. He said the policy's emotional toll including what he called a spike in teen suicides has been traumatic.
The IRS case may take a year or more to build, Karger conceded. "We're really going to dig," he said, adding the filing would be accompanied by "hundreds of boxes of documents," including internal church memos and videos that already have been leaked.
His California-based Rights Equal Rights also has launched a website, MormonTips.com, to encourage LDS whistleblowers.
"That," Karger said, "is where we'll get our best evidence."
Federal law prohibits churches and other tax-exempt entities from participating or intervening on behalf of political candidates.
In past comments, Mormon officials have been adamant they have kept within legally appropriate lines. The LDS Church maintains a neutral stance in partisan matters, but reserves its right to speak out on issues with "significant community or moral consequences."
While motivated by what he sees as the harmful social impact of the LDS Church's LGBT policies, Karger said, his IRS push focuses on political and business activities that he argues have compromised its assertions of remaining on the political sidelines.
He alleges LDS Church involvement in opposing same-sex marriage initiatives in as many as 26 states and the use of Mormon meetinghouses for political organizing.
Karger said he is also working to substantiate assertions that LDS Public Affairs officials assisted Republican Mitt Romney's 2012 presidential bid behind the scenes. Romney, who lost to President Barack Obama, became the first Mormon to head a major party ticket in the U.S.
Karger said he also is targeting taxation on the church's vast commercial and land holdings, including retail operations such as Deseret Book and City Creek Center, the $2 billion shopping complex in the heart of Salt Lake City.
"These activities cross the line," he said, "and nobody has challenged them on it."