Washington(CNN) Before Donald Trump and Steve Bannon were enemies, they were allies. And not long before that, Bannon was part of an effort to sink Trump's presidential hopes -- even if Trump didn't know it.

A conservative watchdog group led by Bannon tried to discredit Trump in the early stages of the 2016 Republican presidential primary by shopping a document alleging that Trump had ties to mobsters, according to conservative sources and a copy of the document reviewed by CNN.

The anti-Trump opposition research was the work of author Peter Schweizer for the Government Accountability Institute, which he cofounded with Bannon in 2012. It described years of alleged business connections between Trump companies and organized crime figures, allegations that have circulated among Trump detractors for years.

The New York Times reported on the document on Friday.

The GAI is backed by the Mercer family, one of the largest benefactors for Trump's campaign. Rebekah Mercer, the daughter of hedge fund billionaire Robert Mercer, is listed as the group's chairwoman on its website. But in 2015, when the document was produced, the Mercers were backing the campaign of one of Trump's rivals, Texas Sen. Ted Cruz, and Bannon had not yet joined the Trump campaign.

In early 2016, at the height of the Republican primary fight, Cruz cited possible mob ties as one reason for Trump to release his taxes. Cruz and his campaign cited published news accounts at the time as the basis for making the charge.

The document offers a glimpse at behind-the-scenes efforts by conservatives to derail Trump's presidential bid. The document is similar to opposition research produced for both Republicans and Democrats targeting Trump. The best known of those is one produced by the Washington firm Fusion GPS alleging ties between Trump and Russians, which now has helped spawn a broad investigation led by special counsel Robert Mueller.

"We research political figures from all political parties and our basic premise is follow the money. That's what guides our research approach," Schweizer told CNN.

A source familiar with GAI's work said the group conducted research on all Republican and Democratic candidates running in the 2016 election. Bannon and the Mercers were not involved in the "day to day machinations of the research," but the source said they were aware of the effort to drill down on candidates and share some of that research with news organizations.

A GOP operative provided CNN a copy of the anti-Trump document. Two sources confirmed that GAI shopped copies of the document to donors for Trump rivals during the GOP primary.

"We did not and would not share that with any candidates," the source familiar with GAI's work said. "There would be no sharing with candidates, with political operatives or anybody of that category."

Bannon declined to comment.

Bannon, Schweizer and the Mercers went on to curry favor with Trump when he became the GOP nominee and, later, the President.

Trump brought on Bannon as CEO of his presidential campaign in August 2016. But Bannon's subsequent West Wing tenure as Trump's chief political strategist was brief. He was fired in August 2017, but remained in contact with the President. Their friendship hit rocky times last week with the publication of comments by Bannon in Michael Wolff's book disparaging Trump and his family.

The President issued a blistering statement against his former political guru, saying Bannon has "lost his mind," and later slapped him with the nickname "Sloppy Steve" via Twitter.

Bannon said Sunday he regretted not responding sooner to comments attributed to him in Wolff's book that were critical of Donald Trump Jr.

It's not clear whether Trump knew of Bannon's and the Mercers' ties to the document aimed at discrediting him when they became his allies in 2016. However, the Mercers' prior support for Cruz was widely known.

Trump regularly cited some of Schweizer's other work on the campaign trail, notably that on Hillary Clinton and alleging corruption. He touted Schweizer's 2015 anti-Clinton book "Clinton Cash", which made use of research by GAI, and urged an investigation of allegations of corruption involving the Clinton Foundation.

The book's allegations formed at least part of the basis for some FBI field offices to open preliminary inquiries into the Clinton Foundation. Those investigations stalled in 2016 amid the election. But CNN reported Friday that the inquiries have been given new life and are now led by the FBI office in Little Rock, Arkansas.