President-elect Donald Trump's defense secretary nominee, retired Gen. James Mattis, has been very clear in recent years that he's not pleased politicians have pushed the military to accept women in combat and openly gay service members.
Asked by McCain whether the military was strong enough to deal with the myriad threats it faces around the world, he emphatically said no.
Separately, Mattis's confirmation hearing also is scheduled for Thursday morning before the Senate Armed Services Committee.
James Mattis added that along with Russia, China and Islamist militants pose the biggest challenge to the US-led world order.
Mattis' hard line on Russian Federation was particularly noteworthy the day after Rex Tillerson, Trump's nominee for secretary of state, faced tough questioning from Republicans and Democrats over his views on the same subject.
Gillibrand repeatedly try to ask Mattis if he meant to roll back the Defense Department's decisions to open combat positions to women and the lesbian, gay, bisexual and transgender community.
But he added that he supported Mr Trump's "desire to engage with Russia". "I'm not sure that's bad, but it's possible". A vote on the waiver is expected by the end of this week.
Before Mattis can join the Cabinet, Congress must approve a one-time exception to a law requiring a military officer to be out of uniform for at least seven years before leading the Pentagon. "The members of our committee have not been afforded a chance to hear from General Mattis, including new members who have just joined Congress and need information to make up their minds".
But Mattis recent military service requires Congress pass an exemption to law created to ensure civilians maintain control of the armed forces.
Mattis also said that Trump must avoid a go-it-alone approach on employing the military and instead work through allies and partnered local forces to carry out missions. Sam Nunn and former Clinton Defense Secretary William Cohen, according to a congressional aide.
"I'm going to urge all House Democrats to vote no on the waiver", said Rep. Adam Smith of Washington state, the top Democrat on the House Armed Services Committee.
The Senate, too, should vote on the measure Friday.
Mattis' written statement also mentioned that he opposes military exchanges or security cooperation involving Cuba, and that he would support attacks on terrorists' families to defeat IS.
Gen Mattis also gave a strong endorsement of the North Atlantic Treaty Organisation alliance that was formed to counter Soviet aggression during the Cold War, which may allay concerns about Mr Trump's comment during his presidential campaign that the United States may not keep its commitment to defend North Atlantic Treaty Organisation allies unless they spend more on their military budgets. "I believe that your appreciation for the costs of war in blood, treasure, and lives and the impact on veterans afterward will enable you to be a check on rash and potentially ill-considered use of military force by a president-elect who perhaps lacks that same appreciation".