His confirmation at stake, Pete Hegseth worked the phones ahead of a U.S. Senate vote late Friday to shore up support to become the nation's defence secretary amid new questions and mounting allegations of heavy drinking and aggressive behaviour toward women.

The Republican-led Senate was determined to confirm Hegseth, a former Fox News host and combat veteran, to round out U.S. President Donald Trump's top national security Cabinet officials, prioritizing his commitment to bring a "warrior culture" to the Pentagon.

But last-minute questions among senators raised fresh uncertainty hours before the vote.

U.S. Vice-President J.D. Vance was expected in the Senate, in the likely event he would be needed to break a tie, and Hegseth himself was at the Capitol with his family to watch the vote.

U.S. Senate Majority Leader John Thune opened Friday's session saying that Hegseth, as a veteran of the National Guard who served tours in Iraq and Afghanistan, "will bring a warrior's perspective" to the top military job.

"Gone will be the days of woke distractions," Thune said, referring to the diversity, equity and inclusion initiatives being slashed across the U.S. federal government. "The Pentagon's focus will be on war fighting."

The Senate's ability to confirm Hegseth despite a grave series of allegations against him will provide a measure of Trump's political power and ability to get what he wants from the Republican-led Congress, and of the potency of the culture wars to fuel his agenda at the White House.