US lawmakers vote Tuesday on a sweeping defence spending bill, the first of three huge measures Congress is scrambling to address this month, but it faces a potential veto from outgoing President Donald Trump.

The $740.5 billion bill's chief Republican sponsor, Representative Mac Thornberry, said strong support could provide a quick victory as Congress turns to approve a federal budget and a potential new relief package for the coronavirus-battered economy.

"The stronger the vote, the less chance of having to deal with a veto later," Thornberry, the top Republican on the House Armed Services Committee, told reporters. The committee's Democratic chairman Adam Smith was more forthright, saying that if the president vetoed the National Defense Authorization Act (NDAA), lawmakers would "come back to vote to override it."

Both chambers are expected to pass the annual defence policy bill for 2021 this week, providing a three per cent pay raise to military personnel. In July, separate House and Senate versions passed by more than the two-thirds supermajority needed to override a presidential veto.

Trump has said he will withdraw his veto threat if lawmakers remove language on renaming military bases that honour Confederate commanders. He is also demanding a provision to revoke a law, known as Section 230, that shields social media companies from liability over what users post on their platforms.

Thornberry said the defence bill should not be held up because of unrelated political disputes. "Section 230 needs to be addressed, but in a different place and a different way," Thornberry said. "Airdropping it at the last minute is not the right thing."

Other Republicans have signalled they are prepared to reject the president's demands. "My inclination would be to always vote for the troops and to vote for our national security," Republican Senator Bill Cassidy told Fox News Sunday.

Another potential sticking point: the NDAA opposes Trump's plan, announced in July, to reduce US troops in Germany. It would bar the Pentagon from reducing its current level of 34,500 troops in Germany until 120 days after a defense secretary review of the action's implications is provided to Congress.

That would delay any troop withdrawal from Germany until after President-elect Joe Biden takes office, on January 20. Reducing US troop numbers there "during a time of growing threats in Europe would constitute a grave strategic mistake that would undermine United States national security interests and weaken NATO," the bill states.