US senators no longer have to follow a dress code when voting on bills or debating in the senate gallery.
Senate Majority Leader Chuck Schumer has directed the sergeant at arms to stop enforcing a requirement that lawmakers wear "business attire".
Senators have long dressed more freely in other parts of the Capitol.
It remains to be seen how many will now swap their suits for yoga pants and Crocs.
"Senators are able to choose what they wear on the Senate floor," Mr Schumer told BBC's US partner CBS News. "I will continue to wear a suit."
CBS News also reported that the relaxed dress code only applies to lawmakers' wardrobes. Staff members and others must continue to wear office-appropriate clothes and shoes in the historic chamber.
The more informal code should make voting easier for Senator John Fetterman, a Democrat who has consistently sported hoodie sweatshirts since returning to work in April from treatment for clinical depression.
To stay within the rules, Mr Fetterman has had to cast votes from the gallery's side doors by turning his thumb up or down, when he is not in a suit.
Republicans have criticised the dress code change on X, formerly known as Twitter.
Representative Marjorie Taylor Greene wrote that the "dress code is one of society's standards that set etiquette and respect for our institutions" and said Mr Fetterman was lowering the bar.
Mr Fetterman responded with a post showing Ms Greene holding up a photo of Hunter Biden, President Joe Biden's son, naked, and chided her about a "higher code of conduct".
The Senate's dress code cannot be found in any publicly posted rule or regulation and seems to be followed generally on the honour system. It has evolved over the decades - women were first allowed to wear trousers in the gallery in the 1990s.
Even before Mr Fetterman arrived in the "most deliberative body in the world", lawmakers pushed the limits.
There was outcry in Washington when then-Senator Hillary Clinton wore a shirt that was deemed too low-cut in 2007.
Senator Krysten Sinema, also a Democrat, presided over the senate in 2021 while wearing a hot pink sweater that read "Dangerous Creature".
During the pandemic, too, Ms Sinema wore brightly coloured wigs, and she frequently sports sleeveless dresses.
The House of Representatives has a more formal dress code that lawmakers modify through voting. In 2019, for example, the House approved allowing members to wear religious headwear such as hijabs.