Senator Elizabeth Warren has earned a rare rebuke by the US upper chamber for quoting Coretta Scott King during the debate on the confirmation of senator Jeff Sessions.

The Massachusetts Democrat ran afoul of the chamber's arcane rules by reading a three-decade-old letter from Dr Martin Luther King's widow written in opposition to senator Jeff Sessions' failed judicial nomination three decades ago.

The chamber is debating the Alabama Republican's nomination for attorney general, with Democrats dropping senatorial niceties to oppose Sessions and Republicans sticking up for him.