Just as the fashion world was turning its attention from London to Milan, Washington made some style news of its own: Chuck Schumer, the Senate majority leader, announced on Monday a relaxation of the “informal dress code” in the chamber.

The outcry was swift, with 46 Republican senators signing on to a letter condemning the shift. “Allowing casual clothing on the Senate floor disrespects the institution we serve and the American families we represent,” they wrote.

But even though the Senate prides itself on a tradition of decorum, expectations of dress in the chamber have been largely governed by norms, rather than by written rules.

That flexibility has allowed for notable deviations from the buttoned-up status quo throughout the years. In the late 1990s, Senator Trent Lott of Mississippi instituted Seersucker Thursday as a nod to the days before air conditioning, when Southern senators introduced their colleagues to the miracle of seersucker, a lightweight, puckered fabric that helped make the muggy Washington summer more endurable. (Seersucker Thursday was also a reminder, according to Mr. Lott, that “the Senate isn’t just a bunch of dour folks wearing dark suits.”)

The most recent guidance, which fell to the Senate’s sergeant-at-arms to enforce, called for business attire while on the Senate floor. For the most part, that has been interpreted by men to mean suits (navy and gray are favorites) and ties (often red or blue, depending on party affiliation). Women have gravitated toward pantsuits or jackets, but a 2019 amendment to the dress code opened the door to sleeveless dresses and was quickly embraced by senators including Kyrsten Sinema.

Shortly after Mr. Schumer’s announcement on Monday, Senator Susan Collins, a Republican who has traditionally favored Chanel-style suits while representing Maine in the nation’s capital, joked that she planned “to wear a bikini tomorrow to the Senate floor.”

Had she followed through, she would have almost certainly taken some of the heat off Senator John Fetterman, Democrat of Pennsylvania, who just months into his freshman term pivoted from suit and tie to hoodie and basketball shorts.

Here’s a look at how the august upper chamber of the United States Congress has evolved from the days of powdered hair to pastel wigs.