Chad Wolf, the acting US homeland security secretary, has become the third cabinet secretary to quit following the storming of the US Capitol by supporters of Donald Trump.
In a message to staff at his department, Mr Wolf said he was resigning because of “recent events”, including legal challenges to his authority to run the department on an acting basis without Senate confirmation.
While he did not refer to the Capitol Hill assault, his resignation follows Mr Trump’s decision to withdraw his nomination to lead the agency after Mr Wolf urged the president to condemn the acts of violence carried out in the president’s name.
“This action is warranted by recent events, including the ongoing and meritless court rulings regarding the validity of my authority as acting secretary,” Mr Wolf wrote. “These events and concerns increasingly serve to divert attention and resources away from the important work of the department in this critical time of a transition of power.”
His resignation comes as an increasingly isolated Mr Trump has disappeared from public view, shorn of his ability to communicate with his 88m Twitter followers after the social network disabled his account.
Mr Wolf’s departure follows the resignation of two other cabinet department secretaries, Elaine Chao at transport and Betsy DeVos at education, as well as a string of departures by administration staffers, in the wake of the assault on the US Capitol.
The attack by an angry pro-Trump mob has prompted Democrats to start impeachment proceedings against the president, who they have accused of inciting an insurrection.
Mr Trump is under more pressure than at any time during his administration following the assault, which came hours after he urged his supporters to head to Capitol Hill to protest the result of an election that he erroneously claims was tainted by voter fraud.
Democrats have called on vice-president Mike Pence to invoke the constitution’s 25th amendment which provides a mechanism to remove a president.
Mr Pence is not expected to respond to the demand, setting the stage for a vote on Wednesday in the Democratically-controlled House of Representatives to impeach Mr Trump.
That would pave the way for a trial in the Senate where Democrats are unlikely to gain support from enough Republicans to convict Mr Trump even though a number of GOP senators have criticised the president over his handling of the situation.
Had Mr Wolf stayed in his job, he would have played a significant role in helping to provide security for the inauguration of Mr Biden on January 20 amid concerns of renewed violence both in Washington and also in state capitals around the country.
Earlier on Monday, Mr Wolf told the Secret Service to begin preparing for the inauguration on Wednesday, a full week before the event rather than a day before as scheduled. US media said that the FBI had warned about the potential for “armed protests” across the US next week.
The resignation of Mr Wolf, whose agency oversees border security, came the day before Mr Trump was expected to fly to Texas to visit the US-Mexico border and tout his limited progress in building the wall that formed a central part of his 2016 presidential campaign.
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