Another prominent former Republican leader has broken his silence on Trump and the Republican Party Thursday, with former House Speaker John Boehner calling for the GOP to "awaken" following Wednesday's "invasion" of the U.S. Capitol.

"I once said the party of Lincoln and Reagan is off taking a nap. The nap has become a nightmare for our nation. The GOP must awaken. The invasion of our Capitol by a mob, incited by lies from some entrusted with power, is a disgrace to all who sacrificed to build our Republic," Boehner said in a tweet.

Rep. Adam Kinzinger, R-Ill., now one of Trump's fiercest critics on Capitol Hill, is the first Republican to publicly call for the invocation of the 25th Amendment to remove him from office.

"Sadly, yesterday it became clear that the president not only abdicated his duty to protect the American people and the people's house, he invoked and inflamed passions that gave fuel to the insurrection we saw here," Kinzinger said in a video posted to Twitter. "When pressed to move and denounce the violence he barely did so, while, of course, victimizing himself."

"All indications are that the president has become unmoored, not just from his duty, or even his health, but from reality itself. It is for this reason that I call for the vice president and members of the Cabinet to ensure that the next few weeks are safe for the American people, and that we have a sane captain of the ship," he added.

Kinzinger also spoke forcefully against the Electoral College challenge -- and his colleagues' support of it -- during the House debate Wednesday night over the ratification of Pennsylvania's vote.

-ABC News' Benjamin Siegel

White House Deputy National Security Adviser Matthew Pottinger resigned on Wednesday, according to a White House official and a person familiar with his resignation, joining a slate of White House staff to resign in the wake of Wednesday's rioting at the U.S. Capitol.

The White House’s National Security Council did not respond to questions about Pottinger, who was an assistant to the president.

-ABC News' Chief White House Correspondent Jonathan Karl and Ben Gittleson

Former Attorney General Bill Barr, once one of President Trump's most loyal allies, is speaking out against his former boss in light of Wednesday's storming of the U.S. Capitol, saying in a statement that his conduct "was a betrayal of his office and supporters."

“Orchestrating a mob to pressure Congress is inexcusable. The President’s conduct yesterday was a betrayal of his office and supporters," he said in a statement to the Associated Press, which was obtained by ABC News.

Barr resigned last month with five weeks remaining in Trump's term after months of growing tensions with Trump that culminated in Barr's refusal to announce investigations into Trump's political opponents and his public rebuke of Trump's baseless claims of widespread voter fraud in the 2020 election.

-ABC News' Alexander Mallin

Rep. Liz Cheney, R-Wyo., a member of House GOP leadership and one of the first prominent Republicans to acknowledge Biden's victory, sent a 21-page memo to her conference making the case against a challenge to the Electoral College certification process on Wednesday, arguing that it would improperly overrule states' authority over elections, and set a "dangerous precedent" down the line for future elections and sessions of Congress.

"By objecting to electoral slates, members are unavoidably asserting that Congress has the authority to overturn elections and overrule state and federal courts. Such objections set an exceptionally dangerous precedent, threatening to steal states’ explicit constitutional responsibility for choosing the President and bestowing it instead on Congress," Cheney wrote.

"This is directly at odds with the Constitution’s clear text and our core beliefs as Republicans. Democrats have long attempted, unconstitutionally, to federalize every element of our nation—including elections. Republicans should not embrace Democrats’ unconstitutional position on these issues," she added.

Cheney raises questions about the proposal from some Republicans to set up a commission to investigate concerns about the election, calling it a "problematic" set up that would take much longer than ten days to wrap up, and potentially delay the inauguration. The memo also exhaustively summarizes the court rulings against the president's legal efforts in the six states Republicans plan to target on Wednesday: Arizona, Nevada, Wisconsin, Michigan, Georgia and Pennsylvania.

-ABC News' Benjamin Siegel