US Senator Kyrsten Sinema from Arizona has announced that she had registered as an independent.
Sinema, previously a Democrat, wrote in an op-ed that she's declaring "independence from the broken partisan system in Washington".
"When politicians are more focused on denying the opposition party a victory than they are on improving Americans' lives, the people who lose are everyday Americans," she said.
Sinema, 46, was elected to the US Senate in 2018 and is now one of three independents serving in the chamber, Xinhua news agency reported.
The announcement came days after Democrats solidified a 51-49 majority in the Senate with Senator Raphael Warnock's win in the Georgia runoff.
Sinema hasn't said whether she will caucus with Democrats as two other Senate independents do.
The White House issued a statement on Friday, saying that Sinema had been "a key partner" on some of the Biden administration's legislative agenda.
"We understand that her decision to register as an independent in Arizona does not change the new Democratic majority control of the Senate," the statement said.
Republicans flipped the House of Representatives in November's midterm elections and have vowed more scrutiny of the White House and the Democratic Party.
The divided, new Congress will convene for the first time on January 3, 2023.
Americans' approval of Congress remains largely negative, with 73 per cent of US adults disapproving of the way the institution is handling its job, according to a new Gallup poll.