In this April 3, 2018, photo, Environmental Protection Agency Administrator Scott Pruitt speaks at a news conference at the Environmental Protection Agency in Washington. (AP Photo/Andrew Harnik)

WASHINGTON - Senate Democrats are formally calling for Environmental Protection Agency chief Scott Pruitt to resign or be fired, citing allegations of ethical lapses and questionable spending on travel and security.

New Mexico Sen. Tom Udall said the mounting allegations against Pruitt mean "it is time for his imperial tenure to end." Udall and other Democrats said they will introduce a formal resolution calling for Pruitt's ouster "for the good of the American people."

The American Federation of Government Employees, a labour union representing about 8,000 EPA employees, has also called for Pruitt to resign or be fired.

Congressional Republicans are largely standing by Pruitt, making it unlikely the measure will succeed in the GOP-controlled Senate. But Democrats say it's a sign of their frustration with Pruitt over his lavish spending on security, first-class airline tickets bought at taxpayer expense and his bargain-priced rental of a $50-a-night Capitol Hill condo tied to a lobbyist.

Speaking at a Capitol news conference, Udall said Pruitt has "misused taxpayer dollars while enhancing his own personal perks," skirted enforcement responsibilities to let polluters off the hook and "subverted scientific processes in a manner we have never seen."

Meanwhile, the Republican-led House Oversight committee has expanded its ongoing review of Pruitt's travel spending to include ethical questions surrounding his bargain condo rental. The property was co-owned by the wife of a prominent lobbyist who runs a firm with a roster of fossil fuels clients who have received favourable regulatory rulings from Pruitt's EPA.

In a letter sent to Pruitt Wednesday, House Oversight Chairman Trey Gowdy of South Carolina said EPA had failed to hand over documents he requested in February, including copies of any waivers permitting the administrator to use premium class airfare. Federal employees are generally required to fly in coach.

The Office of Government Ethics has also demanded documents related to Pruitt's condo rental.

President Donald Trump has defended Pruitt, downplaying the ethical questions swirling around his embattled EPA chief while commending his work at the agency.

"Administrator Pruitt is focused on advancing President Trump's agenda of regulatory certainty and environmental stewardship," EPA spokesman Jahan Wilcox said Wednesday.

EPA Inspector General Arthur Elkins is conducting at least five investigative audits related to Pruitt, including a probe into questionable spending by his swollen security detail. The Associated Press first reported last week that the 20-member team tasked with providing day-and-night protection for Pruitt had racked up salary, overtime and travel expenses approaching $3 million.

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 Tuesday, citing a recent internal EPA analysis that concluded "EPA Intelligence has not identified any specific, credible, direct threat to the EPA administrator."

The career staffer who wrote the February memo questioning Pruitt's security costs, Mario Caraballo, was removed from his job Tuesday shortly after the existence of the memo became public, according to media reports.

EPA acknowledged Tuesday that a personnel action was taken against Caraballo, but said Wednesday that he was still an EPA employee. Spokesman Jahan Wilcox declined to provide details about what specific action was taken against Carballo, the deputy associate administrator of EPA's Office of Homeland Security.

EPA has denied any connection between the personnel action against Caraballo and the release of the Democrats' letter. He was the latest of at least a half-dozen EPA employees dismissed, reassigned, demoted or placed on involuntary unpaid leave after pushing back against the spending demands of Pruitt and his security team.

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News from © The Associated Press, 2018