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 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 spending -- including veterans' care -- topped $1 trillion in the 2023 fiscal year.

"I'm doing this for the warfighters, not the warmongers. The Left is afraid of disrupters and change agents," Hegseth posted on social media, accusing the press of smearing him and vowing to "never back down."

He told CBS as he arrived for a second day of meetings with senators on Capitol Hill that Trump told him to "keep going, keep fighting."

However, 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 confirmation, according to NBC and ABC.

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

The mail, uncovered by The New York Times, came as he was going through a divorce with his second wife, after fathering a child out of wedlock with his third wife.

Penelope Hegseth -- who accused her son of routinely mistreating women for years -- tried to roll back her criticism on Fox News on Wednesday, arguing that she had expressed herself "in haste" and that Hegseth had reformed.

- 'Very disturbing' -

But Graham told CBS News the allegations emerging from Hegseth's past were nevertheless "very disturbing."

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

Trump's nominees can afford to lose the support of only three Republicans at their January Senate confirmation hearings, assuming all Democrats vote against them.

US media have floated various alternatives to Hegseth, with Trump said to be 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 his first TV interview since being nominated, on Fox News.

Betting exchange Polymarket gave him an 83 percent probability of securing the Pentagon post when he was first announced three weeks ago but that has since dropped precipitously, to 12 percent.

Former Florida congressman Matt Gaetz, Trump's first choice for attorney general, withdrew when he was on 30 percent amid his own sexual misconduct allegations.

The New Yorker piled the agony on Hegseth at the weekend, reporting that he was forced to quit roles running two non-profits over allegations of sexual misconduct, heavy drinking and mismanagement of funds.

As many as 10 former colleagues at Fox News have also voiced worries over Hegseth's drinking, NBC News reported.

Hegseth is just one of a number of Trump nominees facing uphill climbs to get through Senate confirmation to join a government set to feature at least three other figures who have denied past accusations of sexual misconduct, including Robert F. Kennedy and Elon Musk.

Former Democratic Hawaii congresswoman Tulsi Gabbard, Trump's pick to be director of national intelligence, is not accused of personal impropriety but has set off alarm bells with her outspoken defense of US adversaries in the past.