The US government fights of hundreds of hacking attacks every day
Barack Obama's administration is continuing its search for a "cyber-tsar", which the US president pledged in May to appoint personally.
The White House's acting cyber-security head, Melissa Hathaway, stood down on Tuesday for "personal reasons".
Senator Susan Collins said Ms Hathaway's departure was a sign of "continued lack of leadership" on cyber-security policy.
A White House spokesman said the selection process was "well under way".
"I wasn't willing to continue to wait any longer because I'm not empowered right now to continue to drive the change," Ms Hathaway told The Washington Post newspaper on Tuesday.
Ms Hathaway, formerly of the Office of the Director of National Intelligence, was temporarily filling the post to accomplish a 60-day review of cyber-security issues ordered by the administration.
Senator Collins said that the loss of Ms Hathaway's expertise was "unfortunate".
"Melissa Hathaway's departure underscores the continued lack of leadership within the Obama administration on cyber-security issues," she said.
Several senior executives are said to have turned down the post already.
White House spokesman Nick Shapiro countered that cyber-security remained prominent on President Obama's agenda.
"The president is personally committed to finding the right person for this job, and a rigorous selection process is well under way," he said.