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ABC News' Matthew Mosk and Brian Ross report:

Underscoring the prominent, if little discussed role that Mitt Romney played as a Mormon leader, the private equity giant once run by the GOP presidential frontrunner carved his church a slice of several of its most lucrative business deals, securities records show, providing it with millions of dollars worth of stock in some of Bain Capital's most well-known holdings.

Romney has always been a major donor to the Church of Jesus Christ of Latter-day Saints, which requires that members "tithe," or give 10 percent of their income to the church. His family charity, called the Tyler Foundation, has given more than $4 million to the church in the past five years, including $1.8 million in 2008 and $600,000 in 2009. But because Romney, whose fortune has been estimated at $250 million, has never released his personal tax returns, the full extent of his giving has never been public.

Newly uncovered stock contributions made during Romney's Bain days suggest there is another dimension to Romney's support for the church - one that could involve millions more than has been previously disclosed.

As part of just one Bain transaction in 2008, involving its investment in Burger King Holdings, filings with the Securities and Exchange Commission reveal that an unnamed Bain partner donated 65,326 shares of Burger King stock to the Church of Jesus Christ of Latter-day Saints, holdings then worth nearly $1.9 million. And there were numerous others, giving the church a stake in other Bain properties, such as Domino's Pizza, the electronics manufacturer DDi, the phosphates company Innophos Holdings, and Marquee Holdings, the parent to AMC Theaters.

The Republican presidential candidate's campaign staff confirmed that some of the stock transactions were at Romney's direction, but they would not say which ones.

"Mitt Romney has publicly stated that he regularly tithes to his church," said Andrea Saul, a Romney campaign spokeswoman, when asked about the Bain contributions. "Some of those church contributions have come through the Tyler Foundation. Others have been donations of stock through Bain. Any shares donated by Mitt Romney are personal shares owned by him."

Saul also noted that not all the shares that appear on Bain securities filings can be attributed to Romney, "as there are other Mormon members of the firm who may also have been making donations to the church of personal shares owned by them."

Questions about Romney's faith have remained largely subdued during the 2012 campaign. Many believe he helped tame the issue during his previous campaign with a December 2007 speech at the George Bush Presidential Library in College Station, Texas, during which he declared that his church would not dictate his actions in the White House, if he was to become President.

"I do not define my candidacy by my religion," Romney said. "A person should not be elected because of his faith nor should he be rejected because of his faith."

Romney said that Mormon church authority is limited to the province of church affairs, "and it ends where the affairs of the nation begin."

The Mormon church is distinct from many other American denominations in what it asks from adherents in money, time and commitment - and not just because it asks young Mormon males to spend two years proselytizing for the faith as missionaries, said Jan Shipps, a religion professor at Indiana University-Purdue University in Indianapolis, and one of the preeminent non-Mormon authorities on the church.

Romney has spoken about the 30 months he spent in France as a missionary, but his role within the church as an adult is largely unexplored. Shipps said Romney has held several significant posts within church leadership, including bishop and "stake" president, a leadership post that covers a sizeable geographic area and requires a significant commitment of time.

Beyond that, Romney appears to have lived up to rigid financial requirements within the church that asks parishioners to contribute 10 percent of their annual earnings.

"People choose their own way of deciding how to tithe," Shipps said. "I know friends who are lawyers who take 10 percent out of their fee. In the 19th century, they would take corn, or wheat. It's up to the person."

Stock contributions, negotiated during his high-wheeling deals while at the helm of Bain Capital, would not be unexpected, she said.

Bain officials said it is common in public offerings, whether in Silicon Valley, on Wall Street, or in private equity, for participants to carve out shares to be donated to a favored charitable cause.

Securities records show that Romney found ways to help include the church in some of the companies most lucrative deals, just as other executives at the firm found ways to generate support for their favored charities. Among the companies named on securities filings as "Bain charitable institution donees" were the Combined Jewish Philanthropies of Greater Boston, The Boston Foundation Inc., Fidelity Investments Charitable Gift Fund, and family foundations run by several top Bain executives.

In some cases the filings are vague about the way stocks are apportioned to the different recipients. In others, such as the 2000 stock sale involving DDi Corporation, the records show the Church of Jesus Christ of Latter-day Saints held 27,016 shares, worth $754,827 at the time of the sale. In a 2008 stock sale involving Innophos Holdings, the church's 50,301 shares were worth nearly $1.4 million. SEC filings for Marquee Holdings note that "certain members and other employees of Bain and its affiliates may make a contribution of shares to one or more charities prior to this offering, including … The Corporation of the President of The Church of Jesus Christ of Latter-day Saints."

Romney's own family nonprofit, The Tyler Charitable Foundation, was also cut into numerous Bain deals. The nonprofit, run by Bradford Malt - the Romney personal attorney who oversees all of the candidate's financial holdings - passed those stock earnings along to a variety of other charities, including the church.