Trump and Pence haven't spoken to one another since last summer, according to sources familiar with their conversations.

Pence defended Trump through a slate of controversies during their administration. But, as the House committee is highlighting at its hearings, Pence drew a line at Trump's alleged plot to overturn the election -- breaking from the president and drawing the rage of the Trump mob on Jan. 6.

When ABC News Chief Washington Correspondent Jonathan Karl interviewed Trump for his book "Betrayal," Karl asked about the "Hang Mike Pence" chants and whether Trump had been concerned for the safety of the man he chose to be his vice president.

“Well, the people were very angry,” Trump said.

“They said, ‘hang Mike Pence,’” Karl told Trump.

“It’s common sense, Jon. It’s common sense that you’re supposed to protect,” Trump said. “How can you, if you know a vote is fraudulent, right, how can you pass on a fraudulent vote to Congress?”

While Pence himself isn't testifying and has not sat before the committee, a range of former Pence aides cooperated with the investigation.

Since his term ended, Pence has publicly reiterated he had no power to overturn the 2020 results. But like other conservatives, he has said "election integrity" should be a national priority.

-ABC News' Katherine Faulders

Notable faces were spotted across the hearing room as proceedings kicked off Thursday.

Capitol Police Officer Harry Dunn, Capitol Police Staff Sgt. Aquilino Gonell, Metropolitan Police Officer Daniel Hodges and former Metropolitan Police Officer Michael Fanone, who testified at the committee's first hearing last year on their experience defending the Capitol on Jan. 6, were all present.

Former Pence national security adviser Olivia Troye, who resigned from the administration in 2020, was spotted sitting next to Gonell as well as Allison Gill, a former high-level Veterans Affairs official who was secretly recording a podcast on the weekends about Robert Mueller's investigation that attracted thousands of listeners.

A couple of members of Congress have been spotted in the back of the room including Reps. Ayanna Pressley, D-Mass., and Steve Cohen, D-Tenn., sitting together. Rep. Adam Kinzinger, R-Ill., who along with Vice Chair Liz Cheney has been ostracized by the Republican Party for speaking out against Trump, also stopped by.

-ABC News' Katherine Faulders and Benjamin Siegel

In his testimony on Thursday, former federal judge Michael Luttig painted a dire picture of what he believed would have happened had Pence followed through with Trump’s plea to remain in power.

“That declaration of Donald Trump as the next president would have plunged America into what I believe would’ve been tantamount to a revolution within a constitutional crisis in America,” Luttig said, “which in my view, and I am only one man, would’ve been the first constitutional crisis since the founding of the republic.”

Luttig is one of the panel's two live witnesses in today's hearing. The former judge informally advised Pence on his role in affirming the results of the 2020 presidential election.

In her opening statement, Vice Chair Liz Cheney played a clip of a Pence pushing back against Trump’s claim that he had the power to overturn the 2020 election in the weeks after the Jan. 6 attack.

“President Trump is wrong,” Pence said in a speech in February before The Federalist Society. “I had no right to overturn the election. The presidency belongs to the American people and the American people alone. And frankly, there is no idea more un-American than the notion that any one person could choose the American president."

Cheney said the select committee will now reveal the details of that pressure campaign.