Ms. Pierson's support in the West Wing began crumbling late Tuesday, in large part because she did not tell the White House of a security failure in Atlanta last month when an armed man was allowed to ride in an elevator with President Obama at the Centers for Disease Control and Prevention.

Despite meeting with the president last week, Ms. Pierson informed him about the incident only minutes before it was reported in the news media on Tuesday evening, officials said.

Josh Earnest, the White House press secretary, said Ms. Pierson's delay in telling the president was a crucial part of "recent and accumulating reports about the performance of the agency" that led Mr. Obama to conclude that the Secret Service needed new leadership.

After Ms. Pierson appeared at a brutal congressional hearing on Tuesday, when she had to explain to a House panel how an armed intruder jumped the White House fence on Sept. 19 and made it as far into the mansion as the East Room, she woke Wednesday to mounting calls for her resignation and withering criticism, some of it from Democrats.