"We face economic challenges that will not pause to let a new president settle in," Bush told a gathering of hundreds of employees from the presidential bureaucracy. "This will also be America's first wartime presidential transition in four decades. We're in a struggle against violent extremists determined to attack us, and they would like nothing more than to exploit this period of change to harm the American people."
The president's comments, under a gray sky on the South Lawn, also had the feel of an early goodbye with 75 days left in office. He stood with his Cabinet, the first lady, and his vice president and his wife, and he grew emotional by the end. His wife, Laura, leaned in to give him a hug.
The White House signaled that after months of staying out of the politics of the 2008 election - often enduring a pummeling from Obama - it would soon start speaking up to defend Bush's record on education, energy, the economy and other issues. The focus will be a natural turn to Bush's legacy.
President-elect Obama and his wife, Michelle, will visit the White House on Monday at Bush's invitation, aides to Obama said.
Meanwhile, the shift from one White House to the next is fully under way, with Bush setting a serious tone and expectations for his staff.
The Bush administration has already arranged security clearances for key Obama transition staffers and is providing working space and policy briefings as well. Career employees, who keep their jobs even when administrations change, have taken on extra work to prevent any disruption in essential services.
"Taken together, these measures represent an unprecedented effort to ensure that the executive branch is prepared to fulfill its responsibilities at all times," Bush said.
"We must keep our attention on the task at hand, because the American people expect no less," Bush directed the executive employees.
Officials at the Department of Homeland Security caution that the U.S. is in a heightened state of alert against terrorism. The fear is that enemies could exploit the transition period to test the country's defenses, as Bush himself warned. No specific threat has been presented to the public.
Preparation for the complex transfer of power has quietly been unfolding for about a year. It accelerated with the landslide election on Tuesday of Obama, the Democratic senator from Illinois.
Obama on Thursday was receiving the first of what will become regular briefings on highly classified information from top intelligence officials.
World leaders are seeking out Obama. White House press secretary Dana Perino said Bush's government is making sure those calls get through.
Bush also suggested there will be no tolerance of pranks during the transition.
When he took office in 2001, many aides found their computer keyboards were missing the W key - a nod to the middle initial in George W. Bush. Staff members of outgoing President Bill Clinton were suspected and criticized for acting immaturely.
Bush told the big gathering of employees on Thursday: "I know that you will continue to conduct yourselves with the decency and professionalism you have shown throughout my time in office."
During the campaign, Obama relentlessly blistered the Bush administration for what he called failed, tired policies that have harmed the country. The White House has sought to make clear that politics will not affect the transition in any way.
The transition involves a delicate dance, in which the White House keeps the president-elect in the loop, and even solicits his input, while decisions still remain solely Bush's to make.
Bush sounded wistful as he looked out at a lot of familiar faces, including some people he's seen at work each day for nearly eight years.
The president recalled that before his inauguration, he said that he and his wife would never quite settle in Washington.
"While the honor is great," Bush said, "the work is temporary."