Pennsylvania Democrat John Fetterman has been sworn in as a senator, to the delight of his supporters.
He is the only new senator who flipped party control of his seat, having won an open seat held by retiring Republican Senator Pat Toomey.
The other six new senators are all replacing retiring members of the same party.
Senator Fetterman, 53, known for his distinct sartorial approach during his time as lieutenant governor of Pennsylvania, ditched his trademark shorts and hoodie and donned a suit.
However, in a tweet posted this morning before he was sworn in on Capitol Hill, he did note that one of his sons carried on the tradition and wore shorts to watch his father take the oath of office.
“For those of you asking, yes, there will be a Fetterman in shorts today, but it’s not me,” he wrote.
His adviser Rebecca Katz tweeted a picture of him with Senator Elizabeth Warren and retweeted a photo of his office nameplate.
Mr Fetterman was officially sworn in on the Senate floor before attending a more celebratory ceremony with Vice President Kamala Harris.
He was accompanied by his wife Gisele Fetterman, who held the Bible on which the oath was taken.
In addition to the couple’s children, also present were Dennis and Lee Horton, Philadelphia brothers controversially convicted for participating in a 1993 robbery and murder, but who maintained their innocence. They were granted clemency at Mr Fetterman’s urging and subsequently joined his campaign for Senate.
Second Gentleman Doug Emhoff tweeted out a picture of the Fettermans recalling the pride he felt when Vice President Harris was first sworn into the Senate six years ago.
The Philadelphia Inquirer reports that on his first day in office, Mr Fetterman stuck close to fellow Pennsylvania Democratic Senator Bob Casey and was greeted by a number of senators from both parties.
The former mayor of Braddock, a small city near Pittsburgh, won 51.25 per cent of the vote statewide in November’s general election, beating Trump-backed Republican candidate Dr Mehmet Oz who received 46.33 per cent.
Mr Fetterman’s winning campaign came even after he suffered a stroke during the Democratic primary.
As he recovered he experienced auditory processing issues and required closed captioning during interviews and debates.
His doctors have said he is still fit to serve.
