Per tradition, the current vice president, Mike Pence, participated in ceremonies for Alabama's Doug Jones and Minnesota's Tina Smith. The new senators were first formally sworn in on the Senate floor, then they each took part in individual ceremonial swearing-in in the Old Senate Chamber.
Former Vice President Joe Biden accompanied Jones, who won a December special election to fill the seat vacated nearly a year ago by current Attorney General Jeff Sessions. Luther Strange was appointed and served during the interim. With Jones' election, Republicans now have a slimmer advantage in the Senate, 51-49.
Biden will speak afterward to a gathering of Jones' friends and family, a person familiar with his plans said.
Walter Mondale, who was vice president during the Carter administration, was also there as Smith took the oath. Smith was appointed to replace former Sen. Al Franken, who officially resigned Tuesday
amid allegations of sexual harassment, most of which were from before he was elected.
Mondale and Biden are both former senators themselves -- Mondale in Minnesota and Biden in Delaware.
While the Senate welcomed in the two new Democrats, it was Biden who appeared to be in the spotlight during their formal swearing-in ceremony on the Senate floor.
The former senator from Delaware greeted Senate Majority Leader Mitch McConnell with a big bear hug before the ceremony.
"It's good to see you," McConnell said with a smile.
A little later, Senate Minority Leader Chuck Schumer joked to Biden loudly while pointing to McConnell, saying "They're going to need your help negotiating." The comment appeared to be a nod to Biden's days of negotiating big fiscal deals with McConnell when Biden was vice president. Congress currently faces another spending deadline on January 19.
Biden worked his way around the room, greeting warmly the roughly dozen Democrats in attendance as well as the five Republicans. Those included Sens. Susan Collins, Thom Tillis, John Barrasso, Chuck Grassley and McConnell. Noticeably absent was Jones' fellow Alabama senator, Richard Shelby, a Republican.
During his ceremonial swearing in in the old Senate chamber, Biden reminded Jones to smile as he got his photo taken.
As president of the Senate, the vice president has a unique relationship with the upper chamber and can cast a tie-breaking vote in the rare event that the Senate is evenly split. Since the 1870s, each vice president has done this fewer than 10 times during their tenure.
Pence has cast six tie-breaking votes so far.
The vice president can also preside over the Senate on ceremonial occasions, as Pence did to gavel in the vote after Republicans passed their final tax reform bill in December.
Pence, a former US congressman and governor of Indiana, frequently visits the Hill as he plays a key role in negotiating the President's legislative agenda.