With all new senators having been sworn in, Majority Leader Mitch McConnell welcomed the new members on the Senate floor.

"To say the 117th congress convenes at a challenging time would indeed be an understatement. From political divisions to a deadly pandemic to adversaries around the world the hurdles before us are many and they are serious, but there are also plenty of reasons for hope," McConnell said.

The Senate also passed a series of administrative resolutions and standing orders for the new Congress -- including an agreement to Wednesday as a the date to count electoral votes. It's a date already set, but this formalizes it.

In his welcoming remarks, McConnell urged members to remember their oath to the Constitution.

"We gavel in today like 116 prior Senates have gaveled in before us with plenty of disagreements and policy differences among our ranks but all -- all swearing the same oath to support and defend the same Constitution," McConnell said. "All loving the same country and all of us committed to do what we can to leave behind an even stronger nation than the one we've been blessed to inherit."

The Senate stands adjourned until Wednesday.

-ABC News' Allison Pecorin

Wisconsin Democrat Rep. Gwen Moore will participate in Sunday’s opening day events –- including the House speaker’s vote, according to her office, after testing positive for COVID-19 just six days ago.

Republicans complained that her presence “certainly seems to be in violation of the (Office of the Attending Physician) quarantine guidance,” and wonder how it was safe for her to travel to Washington after her diagnosis.

A senior Democratic aide did not immediately respond to an inquiry questioning whether special accommodations were being made for Moore, whose office also has not yet answered inquiries regarding how she traveled to Washington following her infection.

One Democratic member, who said he received his first dose of the COVID-19 vaccine last Tuesday, told ABC News that he is “staying clear of everyone,” pivoting to observe that some Republicans are at the Capitol and not wearing masks.

Speaker Nancy Pelosi, whose path to the speakership is narrow with such a slim majority in the new Congress, will depend on Moore’s support to keep her position.

After opening the 117th Congress with the prayer and pledge, the House has initiated a quorum call to establish sufficient attendance ahead of the vote for speaker later in the afternoon. Lawmakers are appearing in groups of 72, arranged by alphabetical order of their last names.

-ABC News' John Parkinson and Benjamin Siegel

The Senate floor is open and Vice President Mike Pence is presiding over the swearing in of the new Senate.

Senators are presenting themselves in pairs -- in alphabetical order and accompanied by the other senator who represents their state -- where a masked Pence is administering the oath to each pair. The first pair of senators sworn in, Sen. Cory Booker, D-N.J., and Shelly Moore-Capito, R-W.Va., each elbow bumped Pence after taking the oath.

The chamber appears almost entirely full and members are applauding after each administered oath.

The Senate will welcome six new members: Cynthia Lummis, R-Wyo., Ben Ray Lujan, D-N.M., John Hickenlooper, D-Colo,, Roger Marshall, R-Kan., and Bill Hagerty, R-Tenn. Sen. Mark Kelly, D-Ariz., was sworn in late last year.

Two Senate seats remain outstanding pending the Georgia runoff elections on Tuesday. Sen. Kelly Loeffler will maintain her seat because it was an appointment, but Sen. David Perdue's term expires as soon as the new members are sworn in.

-ABC News' Allison Pecorin

The 117th session of Congress will begin with both chambers meeting at noon, as required by the Constitution.

New members of the House and Senate will be sworn in under unusual circumstances given the pandemic, with social distancing and fewer family members and supporters in attendance.

Because control of the Senate hinges on the Georgia Senate runoffs, Republicans will begin the new Congress with control of the chamber and a 51-seat majority. The seat of Sen. David Perdue, R-Ga., will be vacant until a winner is formally certified in Tuesday's runoff election.

Sunday afternoon, the House will elect a speaker.

Nancy Pelosi is expected to win with the narrowest of margins, given Democrats’ historically slim majority, but the vote is expected to take longer than last year because of the coronavirus precautions that require lawmakers to enter the chamber in waves.

-ABC News' Mariam Khan and Benjamin Siegel

The House on Monday overwhelmingly voted to override Trump's veto of the must-pass sweeping defense policy bill known as the National Defense Authorization Act.

The final vote was 322-87, receiving the two-thirds majority it required. There were 109 Republicans who voted to override the veto and 20 Democrats voted to sustain it.

The Senate is expected to hold its own veto override vote later this week.

If the Senate also overrides the president's veto, it will be the first time Congress has successfully rejected a presidential veto during Trump's presidency.

Shortly before the vote, GOP Rep. Mac Thornberry of Texas, the ranking member of the House Armed Services Committee, urged his colleagues to vote for “the exact same bill” they did before, emphasizing that “not a comma has changed.”

“I would only ask that as members vote, they put the best interests of the country first,” Thornberry said. “There is no other consideration that should matter."

The bill initially cleared both chambers of Congress with veto-proof majorities earlier this month. Trump then officially vetoed the bill last week because it didn't include a repeal of Section 230, a law that shields internet companies from being liable for what is posted on their websites by them or third parties. The bill also included a provision that would rename military bases named after Confederates, which Trump opposed.

The $740 billion bill includes pay raises for America's soldiers, improvements in body armor for women, coronavirus relief, military housing improvements and boosted sexual harassment prevention and response measures, among other items. It has passed both chambers of Congress for 59 years straight with strong bipartisan support.

Some Republicans voted to sustain Trump’s veto despite supporting the bill earlier this month.

The defense bill must become law before noon Jan. 3, when the new session of Congress begins, or it will expire.

-ABC News' Mariam Khan