Secretary of Homeland Security Janet Napolitano, who was only the third person to head the sprawling department created to safeguard the country from threats ranging from natural disasters to terrorism, will leave the government to head the University of California system.
A former governor of border state Arizona, Napolitano, 55, joined Obama's Cabinet at the start of his administration and was a key player throughout. At one point, she was considered a possible pick for the Supreme Court, and she helped carry out Obama administration policies that drew criticism from immigration advocates who said they deported too many and lawmakers who said they didn't go far enough.
Her departure at the end of the summer comes as Obama tries to move a sweeping overhaul of immigration laws through a Congress at odds over how to deal with the estimated 11 million people who are in the country illegally.
Names of possible successors began floating within hours of her announcement. New York Sen. Charles Schumer called White House Chief of Staff Denis McDonough to make a pitch for New York Police Commissioner Ray Kelly, saying, "We need someone just as good who can fill (Napolitano's) shoes."
White House Press Secretary Jay Carney declined to speculate on any candidates and said Obama would be "very deliberate" in making a choice.
Obama said Friday that he relied on Napolitano's judgment and advice and also came to "value her friendship." He lauded her work, saying her portfolio had included "some of the toughest challenges facing our country." He said she had worked "around the clock" to respond to natural disasters and helped secure the country against terrorist attacks.
"Since day one," he said, "Janet has led my administration's effort to secure our borders, deploying a historic number of resources, while also taking steps to make our immigration system fairer and more consistent with our values."
Napolitano called her four years at Homeland Security "the highlight of my professional career" and said the department had minimized "threats of all kinds to the American public."
She said the agency had improved the safety of travelers and took "smart steps that make our immigration system more fair and focused while deploying record resources to protect our nation's borders."
In addition to being mentioned as a potential Obama Supreme Court choice, Napolitano was considered a potential second term replacement to replace embattled Attorney General Eric Holder, who decided to stay on the job.
Napolitano had her share of critics among immigration advocates and those who want more aggressive border enforcement, but she never became a partisan lightning rod for the administration. Republican Sen. John McCain said his fellow Arizonan had served "with honor" in "one of the toughest and most thankless jobs in Washington."
McCain said the two had had disagreements but that he never "doubted her integrity, work ethic or commitment to our nation's security."
Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, praised Napolitano's work but said her pending departure added to a "growing list of unfilled key leadership positions within the department."
The head of the department's crucial Immigration and Customs Enforcement, John Morton, previously announced plans to step down at the end of July.
Immigrant rights groups gave Napolitano mixed reviews for her tenure. They praised her for supporting a pathway to citizenship for those without documents and with helping with Obama's decision in 2012 to grant temporary relief from deportation to young immigrants who arrived in the U.S. illegally.
The agency also began a policy using prosecutorial discretion to focus on immigrants who posed a threat to public safety, national and border security.
But deportations increased to record numbers under her tenure "and countless families have been torn apart as a result," the Fair Immigration Reform Movement said.
Napolitano made the department – created to marshal widespread security agencies under one roof after the 2001 terrorist attacks _ into a "stronger, more focused and more effective agency," said David Schanzer, a former Democratic staff director of the House Select Committee on Homeland Security and director of the Triangle Center on Terrorism and Homeland Security at Duke University and the University of North Carolina-Chapel Hill.
Napolitano was the first woman to head the security agency, following Secretaries Tom Ridge and Michael Chertoff. Also, James Loy served as acting secretary for two weeks in 2005.
Schanzer said Napolitano's greatest achievements were "tougher enforcement of the southern border and targeting internal immigration enforcement on illegal immigrants who pose the greatest threat, and away from innocent young people who were brought here by their parents."
Sen. Dianne Feinstein, D-Calif., said Napolitano's support for immigration reform, "particularly the bipartisan Senate bill, aided its strong 68-vote passage in the Senate."
Napolitano will be the first woman to lead the 10-campus University of California system. Though she's not an academic and "some may consider her to be an unconventional choice," Sherry Lansing, a University of California regent, called her "without a doubt the right person at the right time to lead this incredible university."
Lansing, a former movie executive and chair of the university's presidential selection committee, said Napolitano "rose to the top" among some 300 potential candidates considered.
"She will bring fresh eyes and a new sensibility -- not only to UC, but to all of California," Lansing said.
Michael Doyle of the Washington Bureau contributed to this report.
Lesley Clark and Michael Doyle
McClatchy Washington Bureau