In particular, Cantwell pressed Bernhardt about his involvement with the Central Valley's Westlands Water District, the nation's largest irrigation district, and Cadiz Inc., which has for years been embroiled in a controversial effort to pump water from a Mojave Desert aquifer.
Both are aggressive players in California water politics and clients of the high-profile lobbying-law firm of Brownstein Hyatt Farber Schreck, where Bernhardt is a partner.
The firm has represented Westlands in four lawsuits against Interior Department agencies. Bernhardt personally lobbied Congress and Interior on behalf of Westlands from 2011 through late last year, during which time the district paid his firm nearly $1.4 million.
Cadiz has paid Brownstein Hyatt $2.75 million in lobbying fees and granted the firm 200,000 shares of company stock with a promise of more if its project to sell desert groundwater to urban Southern California reaches fruition.
Cantwell zeroed in on those relationships, saying, "Westlands and Cadiz represent such large public policy issues with financial interests that it would be better if you recused yourself from [their matters] the entire time that you were at the department."
In a letter to the Interior Department's ethics officer, Bernhardt wrote that if confirmed, he would "withdraw" from his law partnership. He said he would recuse himself from client-related matters for one year — "unless I am first authorized to participate" in them.
"I will follow all of the recusals I have," Bernhardt testified Thursday.
"On top of that," he added, "if I get a whiff of something coming my way that involves a client or a former client for my firm, I'm going to make that item run straight to the ethics office. And when it gets there, they'll make whatever decision they're going to make. And that will be it for me."
Environmentalists argue that given Brownstein Hyatt's extensive list of clients with oil, mining and water interests regulated by Interior agencies, it is likely Bernhardt will receive administration waivers to participate in those matters.
In late March, an acting assistant director at the U.S. Bureau of Land Management revoked two legal guidances that underpinned the agency's 2015 decision that Cadiz could not use an existing federal railroad right-of-way for a new water pipeline.
That decision presented a huge obstacle to Cadiz's potentially lucrative groundwater project. According to federal records, Brownstein Hyatt lobbied Congress and Interior on the matter.
But Bernhardt testified that he didn't discuss the Cadiz project after Trump was elected. "I had no involvement on the Cadiz matter with the [Trump] transition, none with the [Interior] Department, none with the Hill during that period of time."
Committee Republicans praised Bernhardt's Interior experience during President George W. Bush's administration, when Bernhardt rose to the position of solicitor, the department's top lawyer.
Democrats pointed out that some of those years were plagued by scandal.
In 2007, Steven Griles, who had served as deputy secretary, was sentenced to 10 months in prison for obstructing a Senate investigation into lobbyist
The same year, Julie MacDonald, a deputy assistant secretary, resigned after she was found to have revised scientific reports to minimize protections for endangered species.
Committee members peppered Bernhardt with questions about scientific integrity..
"I will look at the science with all its significance and its warts," Bernhardt said. "You look at that, you evaluate it and then you look at the legal decision you can make. In some instances the legal decision may allow you to consider other factors, such as jobs."
He also promised Murkowski, who fought the Obama administration on oil drilling restrictions, that he would work with her state to refill the Alaska oil pipeline.
The committee is expected to vote on Bernhardt's nomination after the Memorial Day recess. It would then go to the full Senate.