Emanuel picked up support from two Republicans to clear the Foreign Relations Committee but the nomination is expected to go down to the wire in the full Senate, where Democrats hold the narrowest of majorities.

Emanuel, who earlier served in Congress and as president Barack Obama's chief of staff, has come under fire over his handling of a Chicago officer's killing of an African American teenager, Laquan McDonald, in 2014, with his administration waiting more than a year to release a police video of the incident.

Two liberal Democrats on the committee, Jeff Merkley and Ed Markey, said they would oppose Emanuel, who was not given any Washington position by President Joe Biden amid a campaign against him by advocates for police reform.

"Black Lives Matter. Here in the halls of Congress, it is important that we not just speak and believe these words, but put them into action in the decisions we make," Merkley said in a statement announcing his opposition.

But Emanuel won support from Republicans Bill Hagerty, a former ambassador to Japan, and Jim Risch, the top Republican on the Senate Foreign Relations Committee.

Risch said he expected Emanuel to fight a proposal within the Biden administration to declare that the United States will never be the first to launch a nuclear attack, only using the ultra-destructive weapons in retaliation.

Such a policy shift "would betray our alliance with Japan" which is "our greatest asset in our strategic competition with China," Risch said.

Some Japanese conservatives have misgivings about a US no-first-use policy, raising speculation in Washington that Tokyo could go nuclear to ensure a greater deterrent.

But many experts doubt there would ever be a decision to develop nuclear weapons in Japan, the only nation that has ever suffered nuclear attack.

Among other nominees, the committee easily moved ahead Nicholas Burns, a veteran US diplomat, as ambassador to China.