Democrats in Congress have opened an investigation after President Donald Trump late Friday abruptly fired State Department Inspector General Steve Linick who, lawmakers said earlier, had been probing Pompeo's alleged misuse of a staffer.
"I've learned there may be another reason for IG Linick's firing," said Eliot Engel, who heads the House Foreign Affairs Committee.
"His office was investigating -- at my request -- Trump's phony emergency declaration so he could send Saudi Arabia weapons."
"We don't have the full picture yet, but it's troubling that Sec Pompeo wanted Linick pushed out," the congressman wrote on Twitter.
Pompeo said in May 2019 there was a state of emergency with Iran, allowing the Trump administration to bypass Congress and sell $8.1 billion in arms to Saudi Arabia and other Arab allies.
The move infuriated lawmakers, who had tried to block the sale on the grounds that the weapons were killing civilians in Yemen, which Saudi Arabia has intensely bombed as it fights Huthi rebels linked to Iran.
Engel and Senator Robert Menendez, the top Democrat on the Senate Foreign Relations Committee, said Saturday that the firing of Linick "may be an illegal act of retaliation."
A Democratic congressional aide, speaking on condition of anonymity, said the inspector general was reviewing how Pompeo and his wife, Susan Pompeo, used a State Department employee.
Linick had heard a complaint that the Pompeos made the staffer walk the family dog, pick up their dry cleaning and make dinner reservations for them, the congressional aide said.
Several Republican senators have also voiced unease about the firing, the fourth time since the beginning of April that Trump has moved to dismiss an official tasked with monitoring governmental misconduct.
In what may have been an initial, defiant reaction to the complaints, Pompeo on Sunday posted a picture of himself, sitting on a porch stoop in a blue polo shirt, cuddling a puppy.
"Introducing... Mercer!" Pompeo wrote on Twitter and Instagram, explaining that his new pet was named after General Hugh Mercer from the US Revolutionary War.
In many countries, few would be surprised that staff is assisting senior government officials in personal tasks but the US system prides itself on transparency.
Pompeo has also raised eyebrows by frequently bringing along his wife on the government plane that takes him around the world even though she has no official role.
In a cabinet of millionaires and billionaires, led by tycoon Trump, Pompeo comes from a comparatively modest background.
He emerged on the political scene only in 2010 when, as the owner of a manufacturing business in Kansas, he was elected to Congress as part of the right-wing populist Tea Party wave.
His 2019 financial disclosure form said he had between $100,000 and $250,000 in his bank account, along with less than $15,000 in a retirement fund.
Pompeo has often cast himself as a voice of middle-class Americans, especially when cutting funding to international organizations such as the UN agency for Palestinian refugees or the World Health Organization.
One of Trump's few aides never to cross him, Pompeo has moved US foreign policy sharply to the right in recent months.
Linick, a veteran prosecutor, was appointed in 2013 by then president Barack Obama but the role is meant to be non-partisan.
In 2016, he criticized former secretary of state Hillary Clinton for using a personal email account to handle State Department business -- an issue seized on by Trump as he defeated Clinton in the presidential race.
Pompeo himself is familiar with congressional probes of the secretary of state. He made his name in Congress with his strident attacks on Clinton over the deaths of four Americans in an Islamist extremist attack in Benghazi, Libya.