At the top of his State of the Union remarks, President Biden acknowledged the changes that came with the 118th Congress, including offering congratulations to McCarthy, the newly elected Republican Speaker of the House.

"I don't want to ruin your reputation, but I look forward to working with you," Biden said after turning around to shake the Republican's hand -- a gesture of goodwill ahead of what is likely to be a two-year period of legislative debate and conflict between Biden and McCarthy's parties.

The president then congratulated the new leader of the House Democrats and the first Black House minority leader in history, Hakeem Jeffries.

Biden then nodded to the longest serving Senate leader in history, Mitch McConnell, as well as Chuck Schumer's latest term as Senate majority leader -- "this time with an even bigger majority."

Biden ended his introduction by lauding former Speaker Nancy Pelosi, someone who he said "will be considered the greatest speaker in the history of this country."

President Biden entered the chamber just after 9 p.m. ET to expected applause from the attendees.

He shook the hands of many lawmakers as he made his way to the podium.

Members began steadily streaming into the House chamber on Tuesday with Vice President Kamala Harris around 8:30 p.m. ET, including some former lawmakers who are also using floor privileges.

The realities of a divided government are on full display -- little conversation between Democrats and Republicans across the aisle on the House floor.

The vice president has been making small talk with House Speaker McCarthy -- overheard on the mics saying it's a "packed house."

Former Rep. Rodney Davis, R-Ill., was seen joking to Rep. Joe Wilson, R-S.C.-- infamous for shouting "you lie!" at former President Barack Obama during a joint address of Congress -- to "sit down, sit down Joe."

Former Department of Housing and Urban Development Secretary Ben Carson is at the speech as one of the only former Trump officials spotted thus far.

Newly independent Arizona Sen. Kyrsten Sinema made her way in, standing next to GOP Utah Sen. Mitt Romney and was then speaking with Republican Senate Minority Leader Mitch McConnell.

For some, the most coveted seats appear to be along the aisle. That's where embattled Rep. George Santos chose to sit, in position to shake the president's hand as he enters and exits the chamber. Santos still has time to switch seats.

Some of the president's fiercest critics are also along the aisle: Republican Reps. Matt Gaetz and Lauren Boebert.

Rep. Marjorie Taylor Greene was carrying around a white balloon through the halls of Congress for much of the day to mock the administration's handling of the suspected Chinese spy balloon -- but decided to not bring it into the chamber.

Speaker McCarthy's guest, Enes Freedom, New York City Mayor Eric Adams, Cabinet secretaries Pete Buttigieg and Jennifer Granholm and pollster Frank Luntz have also filed into the room.

-ABC News Senior Congressional Correspondent Rachel Scott, Katherine Faulders and John Parkinson

A moment between embattled Rep. George Santos and Sen. Mitt Romney ahead of Biden’s speech is catching attention on social media.

The two Republicans appeared to be exchanging insults on the House floor before the president's arrival.

"You should be embarrassed ... you shouldn't be here," Romney seemingly said to Santos, who was sitting off the center aisle of the chamber.

The two went back and forth again before Romney walked away.

On another feed, Santos can be seen repeatedly saying "what an a------" to his seat mate, Rep. Thomas Massie, R-Ky., in apparent reference to Romney.

-ABC News' Justin Fishel and Ben Siegel

As he reupped his "Unity Agenda" -- aimed at improving mental health, supporting veterans, beating the opioid epidemic and fighting cancer -- Biden shared personal stories of some of the guests in the audience.

"Joining us tonight is a father named Doug from Newton, New Hampshire," Biden said. "He wrote Jill and me a letter about his daughter Courtney. Contagious laugh. Her sister’s best friend. ... Courtney discovered pills in high school. It spiraled into addiction and eventually her death from a fentanyl overdose. She was 20 years old."

Biden said her family's been working since her death to end stigma and change laws. "Doug, we're with you," he said.

He next introduced Maurice and Kandice, whose daughter Ava was diagnoses with a rare kidney cancer when she was 1 year old. Ava had to undergo 26 blood transfusions, 11 rounds of radiation, 8 rounds of chemo and had one of her kidneys removed, Biden said.

"They never gave up hope," Biden said. "Ava never gave up hope. She turns 4 next month. They just found out that Ava beat the odds and is on her way to being cancer free, and she’s watching from the White House tonight."

The president, whose oldest son, Beau, died of cancer, has made working to end the disease a major health priority of his administration.