Pete Hegseth could soon face impeachment proceedings as Democrats plan what to do with their majorities in both houses of Congress.
Also likely to face congressional subpoenas are Elon Musk and Donald Trump's new chief of staff Stephen Miller.
Democrats gained control of both chambers in the midterm elections this week, flipping states that have voted Republican for decades.
California representative Robert Garcia, who is in line to chair the powerful House Oversight Committee next year, said investigations will focus on Mr Musk's involvement in the Department of Government Efficiency's firing of scores of federal workers last year.
The part played by Mr Miller in Trump's hardline immigration crackdown, which included operations in which two people were fatally shot in Minnesota by federal agents, will also come under intense scrutiny, said Mr Garcia.
"I think there's a lot of interest, not just from our committee, but from committees across the Congress to speak to
Mr. Musk," Mr Garcia told CNN's State of the Union.
The lawmaker noted Mr Miller is at the centre of major policy decisions in the administration, so "oversight and investigation" are due.
The suggested probes of Mr Musk and Mr Miller are among the first indications of the Democrats' plans for January, when they will be seated.
House Democratic leaders are also placing an early emphasis on healthcare, and vowing swift action on legislation to stop the Iran war and to reverse some of Mr Trump's trade tariffs, with some also promising to push for clean energy.
A Reuters/Ipsos poll in October showed that only 37% approved of Mr Trump's handling of immigration, down from 50% when he took office.
With Mr Miller installed in his position, the Trump administration is expected to continue its aggressive approach to immigration enforcement, as well as stoking cultural conflicts and standing firm against any moves made by the Democratic-controlled Congress, say experts and officials.
Hakeem Jeffries, who will become speaker of the US House in January, said on Sunday that Democrats would support a measure to impeach Pete Hegseth, a congressional process that could see him removed from office, although that would require 67 votes in the 100-seat Senate, where the Democrats currently have 54 seats.
He told ABC News he's a "hard yes" on the impeachment, adding that Mr Hegseth has "abused his power" and "undermined our national security".
Mr Jeffries had asked in a social post on Thursday: "Why is Pete Hegseth still around?"
The defence secretary issued a more than four-minute video message on social media that day, vowing to fight efforts in Congress to impeach him.
He defended the Iran war as "courageous and historic" after voters punished Republicans over the conflict in this week's midterm elections, as taunts from his enemies in Congress escalated.
Mr Trump has batted away questions about whether he might fire Mr Hegseth, but has previously commended his Pentagon chief.
On Saturday, the US president reposted the video on Truth Social, without offering any explanation for doing so.
Mr Hegseth, a former Fox News host, has been a polarising figure, even within his own Republican party.
Democrats have promised to investigate Mr Hegseth and his handling of the Pentagon, with congressional aides saying this would include his firing of the chairman of the joint chiefs of staff, among other issues.
"Pete Hegseth has got to go... I believe there will be incredibly strong support for that resolution," Mr Jeffries told
ABC's This Week, citing his oversight of the Iran war, which has led to a surge in gas prices and vice president JD Vance said cost Republicans control of both chambers.
Republican Representative Thomas Massie promised to reintroduce articles of impeachment against Mr Hegseth next week, warning that impeachment would be "much gentler" than what is set to come his way once Republicans cede control of Congress next year.
"They'll have hearings & trials," Mr Massie wrote on X.
(c) Sky News 2026: Pete Hegseth could face impeachment proceedings as Democrats also target Elon Musk and S
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