Hegseth, a former Army National Guard officer and Fox News weekend presenter, is under intense pressure over a series of misconduct allegations, including accusations of alcohol abuse and a sexual assault claim from 2017, over which no charges were filed.

Hegseth denies all wrongdoing but the controversy has left Trump's transition officials scrambling to avoid the embarrassment of a second Cabinet nomination collapsing amid dwindling support from Republicans in Congress.

Running the Pentagon is one of the biggest roles in public office. The Defense Department employs almost three million military and civilian staff, and defense spending -- including veterans' care -- topped $1 trillion in the 2023 fiscal year.

Up to six Senate Republicans -- including South Carolina's Lindsey Graham, one of Trump's staunchest allies on Capitol Hill -- have voiced doubts over Hegseth's ability to walk the tightrope to confirmation, according to NBC News.

The questions around Hegseth's character deepened as an old email emerged in which Hegseth's own mother called him an "abuser of women."

"I think some of these articles are very disturbing," Graham told CBS News of the media coverage around the nomination.

"He obviously has a chance to defend himself here, but some of this stuff is going to be difficult."

Republicans will have 53 seats in the incoming Senate majority, meaning Trump's nominees can afford to lose the support only three Republican votes at their January confirmation hearings, assuming all Democrats vote against them.

US media have floated various alternatives, with Trump said to by mulling one-time Republican primary rival and Florida Governor Ron DeSantis.

The move would raise eyebrows in Washington, as the pair had only the most perfunctory of reconciliations after a bitter presidential nomination battle that left both bruised, although the governor did endorse Trump after dropping out.

On what is being seen as a day of reckoning for Hegseth, the 44-year-old was due for further meetings with influential Republicans on Capitol Hill, and his first TV interview since being nominated, on Fox News.