In a hearing on his appointment as the agency's deputy administrator last November, Wheeler described himself as someone who "always tried to find common ground and work across the aisle" in Washington.
Meanwhile, activists argued Mr Pruitt's policies of moving to boost fossil fuel production and rollback regulations opposed by corporate interests had endangered the environment.
Excluded from Pruitt's official schedule were meetings with an energy lobbyist, the CEOs of a shipping company and a coal company, and Cardinal Pell, who was subsequently charged with criminal sexual offenses, the Democrats said.
Trump announced that Pruitt's deputy, former coal lobbyist Andrew Wheeler, would take over Monday as acting head of the agency that the president has tasked with dismantling Barack Obama's green legacy. He worked in the EPA under former President George H.W. Bush, spent years as a senior aide in the Senate working to defeat climate-related legislation and most recently was a lobbyist with many clients in the coal industry.
In the letter, Carper also recommended a series of actions he believes Wheeler should take to "restore confidence" in the agency after Pruitt's tenure. But President Trump appears to be standing by his EPA administrator based on Pruitt's commitment to weakening and cutting environmental regulations.
Environmental Protection Agency Administrator Scott Pruitt testifies before the House Energy and Commerce Committee's Environment Subcommittee in the Rayburn House Office Building on Capitol Hill on April 26 in Washington, D.C. The focus of almost a dozen federal inquiries into his travel expenses, security practices and other issues, Pruitt testified about his agency's FY2019 budget proposal. The recommendations include a 50 percent cut to the EPA staff, and US withdrawal from the Paris climate change agreement-which Trump already initiated.
In a statement, Tim Huelskamp, president and CEO of the conservative Heartland Institute said, "Scott Pruitt has been the most effective member of Trump's administration despite the controversies he generated".
Trump had praised Pruitt for his regulation-trimming ways at EPA. Trump needs to nominate a new agency chief, and his choice would then have to be confirmed by the Senate, which may not happen by mid-term elections in November.
The growing ethical cloud around Pruitt ultimately grew too big for the Trump administration to handle, and Pruitt resigned on Thursday.
"The damage Scott Pruitt has done to the Agency will not easily be undone", he wrote. "So when you're weakening these types of things, you're putting more people's lives at risk", Ali said.
Some of the ethics accusations against Pruitt also involved jobs for his wife.
In one of the investigations, the U.S. Government Accountability Office concluded that the EPA violated two laws by installing the $43,000 phone booth for Pruitt's office without telling lawmakers first.
Some of the tasks were outright freaky, including a reported order to find and obtain a used Trump-brand mattress from a Trump hotel.
Wheeler's and others' lobbying efforts worked: Late past year, President Donald Trump traveled to Utah to officially announce the reduction of Bears Ears.