Senate Democrats have called for the “imperial tenure” of Environmental Protection Agency (EPA) head Scott Pruitt to come to an end after reports of his travel and other spending came to light.

Congressman Tom Udall also said Mr Pruitt’s departure would be “for the good of the American people” after spending taxpayer dollars on unnecessary first-class plane tickets, security measures deemed excessive, and his rental property in Washington DC which is curiously low-priced at $50 per night and owned by a lobbyist.

EPA spokesman Jahan Wilcox said "Administrator Pruitt is focused on advancing [the president's] agenda of regulatory certainty and environmental stewardship”. President Donald Trump also came to the aid of Mr Pruitt. He tweeted on 7 April: “While Security spending was somewhat more than his predecessor, Scott Pruitt has received death threats because of his bold actions at EPA. Record clean Air & Water while saving USA Billions of Dollars. Rent was about market rate, travel expenses OK. Scott is doing a great job!”

EPA Inspector General Arthur Elkins is conducting at least five investigative audits related to Mr Pruitt’s spending on travel and security. The Associated Press first reported that his security detail - a whopping 20 members compared to his predecessor Gina McCarthy’s small team which only drove her to and from the office or accompanied her on certain trips - cost taxpayers close to $3m after taking into account salary, overtime, and travel.

Mr Pruitt's staff has said the outsized security spending and his first-class flights were justified because of "unprecedented" death threats against him. Senate Democrats sought to undercut that argument, citing a recent internal EPA analysis that concluded: "EPA Intelligence has not identified any specific, credible, direct threat to the EPA administrator."

A career employee who had written the memo regarding the administrator’s expensive, Mario Caraballo, was sacked just days later. At least six other staffers have been dismissed, demoted or placed on involuntary leave after pushing back against Pruitt's spending demands.

Mr Udall was speaking at a news conference and said the president’s appointee on environment had “subverted scientific processes in a manner we have never seen”. Under Mr Pruitt, the EPA has essentially disbanded a major scientific advisory committee and caused the resignation of several scientists from other advisory positions. One, in particular, told The Independent previously that he refused to be a “future prop for bad science”.

Mr Pruitt has also replaced departing scientific advisors with industry professionals, "who understand the impact of regulations on the regulated community,” according to an EPA spokesperson who spoke on the matter last May.

The EPA is also set to reach its lowest number of staff in three decades under Mr Pruitt’s reign. In March 2017, Mr Trump proposed cutting the agency’s budget by nearly a third, or about 3,200 jobs according to the newspaper. At least 1,200 people in those jobs were offered a buyout in June 2017 and a third of them - or 400 employees - took it or retired as of 31 August. If more employees take the buyout later in the month or retire, the EPA’s employment would drop to 14,400 people which is the lowest it has been since 1998.

The Office of Government Ethics is also demanding documents related to Mr Pruitt's condo rental after it was revealed the property is co-owned by the wife of a lobbyist with several fossil fuels clients, some of whom have received beneficial regulatory rulings from the EPA under Mr Pruitt.

Republicans on Capitol Hill have argued that Mr Pruitt has helped reign spending agency-wide and reduced federal environmental regulations in order to allow state agencies to decide what is best for them.

Agencies contributed to this report.