Kentucky congressman Thomas Massie introduced eight articles of impeachment accusing Hegseth of illegally waging war without congressional authorisation and abusing his powers as Pentagon chief.
The Republican-controlled House could vote as early as Wednesday, with some lawmakers from Trump’s party signalling that they might join Democrats in supporting the charges.
Impeachment would be a humiliating blow for Hegseth and the Trump administration, but would not automatically remove him from office.
That would require a trial and conviction by a two-thirds majority in the Senate – a far higher hurdle, even after Democrats take control of the chamber in January.
The impeachment push has gathered momentum since last Tuesday’s midterm elections, when Democrats swept both chambers of Congress amid widespread anger over the Iran war and soaring fuel prices.
Republicans retain control of Congress until the newly elected lawmakers take office on January 3, but their overwhelming defeat has fuelled recriminations and weakened party discipline.
Some Republican lawmakers who lost their seats could also skip votes, potentially making it easier for the impeachment measure to pass.
Massie, an outspoken critic of Trump who lost his own primary to a candidate backed by the president, is using a special congressional procedure to force the House to consider his resolution within two legislative days.
Republican leaders could still seek to block the measure through a procedural vote.
The articles accuse Hegseth of violating the 1973 War Powers Resolution, which limits the president’s ability to engage US forces in hostilities without congressional approval.
They also allege misconduct involving civilian casualties and military operations in Yemen and Venezuela.
The Pentagon has rejected the accusations.
Democratic House leader Hakeem Jeffries, who is expected to become speaker in January, threw his weight behind impeachment on Sunday.
“I’m a hard yes on the Hegseth resolution,” Jeffries told ABC News.
“The so-called secretary of defence has abused his power, undermined our national security and engineered this reckless and costly war of choice in Iran,” he said.
“Pete Hegseth has got to go.”
Several Republicans have expressed dissatisfaction with Hegseth’s leadership without committing to impeaching him.
Arkansas congressman French Hill said on Sunday he wanted to hear the debate before deciding whether the accusations justified impeachment.
Under the US Constitution, the House can impeach a senior government official by a simple majority vote, effectively bringing formal charges of serious misconduct.
The Senate then decides whether to convict and remove the official from office, requiring support from two-thirds of senators present.
Even with their enlarged majority next year, Democrats would need substantial Republican backing to reach that threshold. Only two Cabinet secretaries – almost 150 years apart – have been impeached in US history, and neither was removed by the Senate.
Hegseth has dismissed Massie’s impeachment effort as a “sham,” calling the congressman “disgruntled” and accusing his opponents of pursuing political destruction.
The Pentagon announced a rare press briefing by Hegseth scheduled for Tuesday.
Trump has publicly stood by his embattled defence chief, declaring last week: “I have every intention to keep him.”
But the president’s weakened political position after the midterms has left Hegseth facing the prospect of an impeachment battle that could extend well beyond this week’s House vote. (AFP)
