'I don't know why the right side seems to be losing their minds over it,' the Democratic senator from Pennsylvania told Fox News.
Senate Majority Leader Chuck Schumer decided to no longer enforce an official 'business' dress code for members of the Senate, which many saw as a concession to Fetterman, 54, who is often seen wearing shorts and a hoodie.
'I think it's a good thing, but I'm going to use it sparingly,' Fetterman said. 'I hope other colleagues take advantage of it too.'
'I don't know why the right side seems to be losing their minds over it,' Democratic Senator John Fetterman of Pennsylvania said of Republican criticism of the new Senate dress code
While there is no formal dress code, men are expected to wear a jacket and a tie and women to wear business clothes. While on the Senate floor, the lawmakes are visible to the television cameras that broadcast the Senate's business online and on C-SPAN.
Previously, if a senator was not wearing clothes that met the requirements, he or she would have to shout 'Aye' or 'Nay' from the doorway of the chamber so they weren't visible to the cameras.
When Fetterman reached the Senate floor on Monday night, casually dressed, he still voted from the doorway, where the cameras could not catch him. 'Baby steps,' he told reporters as he got on the elevator to go back to his office.
And some senators took advantage of the new rule Monday, when they arrived back in Washington D.C. for a vote in the Senate that evening.
'I would not normally wear this on a Tuesday, Wednesday or Thursday,' Murkowski told reporters. 'I mean, it's a respect thing, it's like going to church in your jeans, or going to a funeral in jeans.'
'I'm not so hung up on things to think that every single day a man needs to wear a necktie,' she added.
Connecticut Sen. Chris Murphy, a Democratic, did opt to go tieless on Monday evening, noting in the past he would have been reprimanded by Sergeant-at-Arms staff.
Republican Sen. Susan Collins of Maine mocked the new dress code, saying: 'I plan to wear a bikini tomorrow to the Senate floor.'
Democratic Senator Amy Klobuchar of Minnesota took advantage of the new dress code to wear black pants, running shoes and a casual jacket; she talks to Senator Mitt Romney on the Senate floor Monday night
Republican Senator Lisa Murkowski - traveling from Alaska - wore black pants, running shoes and a pink workoutshirt to votes on Monday evening
But other senators said they would stick to a formal way of dressing even if meant going without some of their favorite accessories.
Republican Senator Chuck Grassley, who wears a bright green John Deere baseball hat back home in Iowa, said he wouldn't don one in the Senate.
'What has been suggested stinks, it's unprecedented, and if you can't count on the majority leader in the United States Senate to uphold the decorum of the United States Senate, who can you call on?,' he said.
Not all Republicans were upset about the change.
'Now I can vote from the Senate floor on Mondays,' Hawley said, noting that he usually wears a suit and tie every other day.
But it's Fetterman who has taken the heat for the new way of dress. Republican Rep. Marjorie Taylor Greene slammed him on social media: 'The Senate no longer enforcing a dress code for Senators to appease Fetterman is disgraceful.
'Dress code is one of society's standards that set etiquette and respect for our institutions. Stop lowering the bar!'
Fetterman told CNN, jokingly, the criticism was 'devastating.'
'It's mystifying. I mean, there's certain much more important kinds of issues we should be addressing. Instead of like, how if I dress like a bum,' he said.
The senator has faced a number of health challenges, including being in hospital for lightheadedness and clinical depression earlier this year. He is still said to suffer auditory processing issues stemming from a stroke he experienced in May 2022.
Senate Majority Leader Chuck Schumer confirmed on Sunday that he had directed the Senate's sergeant-at-arms to stop enforcing the dress code for senators.
He did not mention Fetterman in his statement about the dress code, which will only apply to senators, not staff.
The change has humorously been dubbed the 'Fetterman Rule' due to Fetterman's preference for a far more casual wardrobe.
Schumer later clarified in a statement that senators can now choose their attire on the Senate floor, though he said he personally intended to continue wearing a suit.
'There has been an informal dress code that was enforced,' Schumer said in a statement. 'Senators are able to choose what they wear on the Senate floor. I will continue to wear a suit.'
Republican Senator Chuck Grassley of Iowa , who wears a bright green John Deere baseball hat back home, said he would never wear it in the Senate
Senator John Fetterman, seen above in May, has worn a suit and tie in the Senate
'Senators are able to choose what they wear on the Senate floor. I will continue to wear a suit,' Senator Chuck Schumer said in a statement
However, there has also been outrage from some of the chamber's other lawmakers
Kansas Sen. Roger Marshall, a Republican, said 'It's a 'sad day in the Senate' and that the people who Fetterman and Schumer represent should be embarrassed.
'I represent the people of Kansas, and much like when I get dressed up to go to a wedding, it´s to honor the bride and groom, you go to a funeral you get dressed up to honor the family of the deceased,' Marshall said. Senators should have a certain level of decorum, he added.
It's unclear if the rules for more formal attire were actually written down anywhere, but Schumer´s directive means that staff will no longer scold senators for their choice of clothing or ask them to vote from the doorway.
'I think we should all want to be more comfortable,' Fetterman told a group of reporters on Monday. 'And now we have that option, and if people prefer to wear a suit, then that´s great.'