"Janet Napolitano offers of the experience and executive skills we need in the next secretary of homeland security," Obama said during a news conference in Chicago. "She understands the need for a Department of Homeland Security that has the capacity to help prevent terrorist attacks and respond to catastrophe be it manmade or natural."

Noting she is capable of reforming the Homeland Security Department while protecting the United States, Obama said Napolitano "understands as well as anyone the danger of an unsecure border."

Napolitano said the job of the national security team Obama introduced Monday would be "to hold ourselves and our agencies accountable, to coordinate fully across the spectrum of government agencies and to ensure that we work hand in hand with state and local governments to share information, secure our borders and keep our country safe."

Napolitano said leaving the governorship would be difficult, "but one must go where one can best serve."

In an e-mail, Napolitano told Arizonans she intended to proceed with her state responsibilities, including delivering the state-of-the-state address and presenting a budget to the state Legislature.

"Upon confirmation as secretary by the U.S. Senate, I will tender my resignation as governor of Arizona," she wrote. "I will continue to work for Arizona and Arizonans, but from a different place and with a different charge."