(CNN) As a former fossil fuels lobbyist who now leads the Interior Department, David Bernhardt has oversight of the industry he used to represent.

While Bernhardt has recused himself for a period of time from certain dealings with his former clients, he serves as one of the main architects of the Trump administration's deregulation agenda that has benefitted energy companies -- including some of Bernhardt's former clients.

Since Bernhardt joined the department in 2017, it has made at least 15 policy changes, decisions or proposals that would directly benefit Bernhardt's former clients. In each case, the past clients shared comments or requests that the department's actions have granted or aligned with, according to a CNN review.

In some cases, the former clients wrote letters addressed to Bernhardt about those issues or lobbied his colleagues before those colleagues met with Bernhardt on those matters, according to documents.

President Donald Trump has said he will nominate Bernhardt to succeed Ryan Zinke as Interior secretary.

A spokesperson for Bernhardt said in a statement that he has fully complied with his recusal agreements, and some advocates for the energy industry argue that such regulatory rollbacks are part of what Trump was elected to do.

But some environmental groups charge that such decisions by Interior under Bernhardt amount to "favors" for his former clients.

"We're seeing a raft of favors both large and small being granted, everything from specific projects getting greenlights to larger policies that are either being rolled back from the Obama administration or moved forward in the Trump administration," said Jesse Prentice-Dunn, of the Center for Western Priorities, a conservation group that catalogued some of the Interior actions requested by Bernhardt's former clients.

Bernhardt, who has devoted himself to understanding the intricate details of environmental regulations, has bounced back and forth between government service and lobbying for industry groups over the course of his career.

After working for a Colorado member of Congress in the 1990s, he joined a lobbying firm and later held several positions at Interior from 2001 to 2009 during President George W. Bush's administration, including as solicitor. Bernhardt left the department to head the natural resources law division of the firm Brownstein Hyatt Farber Schreck.

When he re-entered Interior as its deputy secretary for the Trump administration, Bernhardt signed an ethics pledge that recused him from certain matters directly involving 26 former clients, including Halliburton Energy Services, water-supply company Cadiz, Inc., and the offshore-energy trade group National Ocean Industry Association. The recusals for four of those clients have already expired, and the rest will expire in August.

An Interior spokesperson for Bernhardt, Faith Vander Voort, told CNN that his recusal agreements require him not to substantially engage "in any particular matter involving specific parties" related to his former clients for two years after his appointment. That means Bernhardt cannot work on issues such as lawsuits, permits, grants or agreements that directly involve his former clients and generally cannot meet or communicate with those former clients during that time, unless he gets a waiver.

Vander Voort said any critics who suggest Bernhardt has crossed ethical lines with his former clients are either incorrect or deliberately misleading, adding that some may conflate acceptable types of interactions with the "particular matters" Bernhardt is recused from.

His recusal list means former clients are lobbying officials around Bernhardt, according to a CNN review of emails and documents.

For example, Bernhardt's former client the Independent Petroleum Association of America repeatedly met with and emailed Interior officials about the sage grouse, a western bird that the Obama administration sought to protect with a plan that blocked oil and gas companies from building on much of the birds' habitat.

In July and August 2017, IPAA lobbyist Samantha McDonald emailed an Interior staffer about the agency issuing new guidance on maps related to sage grouse and the staffer responded that Interior was "working on it," according to emails obtained by the Western Values Project.

Public calendars show Bernhardt attended at least three staff meetings related to the sage grouse, at least one of which included staffers who communicated with IPAA on the issue or met with the group's representatives.

In a March 2018 letter, IPAA and other groups specifically thanked Bernhardt for Interior's regulation rollbacks and said they looked forward to working with the agency on revising sage grouse land-use policies.

In December, the Trump administration released a plan to amend the Obama-era sage grouse protections and open vast swaths of western land for mining and drilling.

The IPAA also praised Bernhardt in a 2017 letter for helping to reform the National Environmental Policy Act and asked that Interior reinstate "categorical exclusions," which allow the agency to streamline the analysis required to approve projects that could affect the environment, which can save the energy industry time and money.

Bernhardt later authored a memo stating that categorical exclusions should be "fully and properly" considered before such reviews.

A spokesperson for IPAA, Jeff Eshelman, said the association has not had any personal contact with Bernhardt since he joined Interior and that IPAA has lobbied Congress and administrations with these policy recommendations for decades.

"Our policy positions have not changed, our outreach efforts have not changed based on who is in control of Congress or the Administration," he said.

Other Interior decisions that reflect comments or requests from Bernhardt's former clients include the rescinding of Obama-era rule that set limits on hydraulic fracturing on federal land and the rollout of plans to expand offshore drilling.

Some of the requests were highly specific. Bernhardt's former client Noble Energy, for example, commented on an Interior proposal and said the time-period to submit certain documents related to safety systems during an application process should be extended from 60 days to 90 days. A final rule issued in 2018 included that extension.

Interior also cleared the way for one of Bernhardt's former client's projects in California.

Cadiz Inc., a water-supply company, proposed a pipeline running water from the Mojave Desert through land owned by the Arizona and California Railroad. The Obama administration blocked the project and stated that an 1875 law mandated that the land be used for railroads.

But an Interior letter sent in 2017 to Scott Slater, Bernhardt's former colleague at Brownstein Hyatt Farber Schreck and the CEO for Cadiz, disagreed and stated , "There is no evidence that the Cadiz project would interfere with continued use of the easement for railroad operations."

Myron Ebell, the director of the Center for Energy and Environment at the Competitive Enterprise Institute, a free-market organization, argues such project approvals and regulation rollbacks have occurred not because of Bernhardt's position in office but because of the Trump administration's broader efforts to promote energy independence and growth.

"Dave Bernhardt is not pursuing his agenda or that of any people he worked for as a lawyer. He's pursuing the agenda President Trump won the election on and is implementing," Ebell said.

Still, some Democratic lawmakers have expressed concerns about Bernhardt's interactions with the energy industry.

A letter by Rep. Raul Grijalva, who chairs the House Committee on Natural Resources, states more than 100 hours of official government time on Bernhardt's schedule did not include descriptions or list the non-Interior attendees. The letter requests all calendars maintained by Bernhardt.

Sen. Elizabeth Warren (D-Massachusetts) and Sen. Richard Blumenthal (D-Connecticut) asked Interior's inspector general to investigate Bernhardt's involvement in proposals that would revise rules protecting the delta smelt, a California fish.

The New York Times reported that Bernhardt, while at Interior, has worked to strip away such rules, which are also opposed by his former client Westlands Water District, a state-chartered organization that represents farmers who want access to the river water where the fish dwells.

Prentice-Dunn, with the Center for Western Priorities, said Bernhardt's position at Interior marks another example in the Trump administration, in addition to Andrew Wheeler who heads the EPA, of a lobbyist ascending to lead a regulatory agency despite Trump's campaign promise to "drain the swamp."

"Bernhardt has spent his career trying to work the levers of power. Now he is in control of those levers," Prentice-Dunn added.