WASHINGTON — With so much to do and dwindling time left to do it, President Bush is turning to his most popular surrogate — the first lady — to help make in-roads in policy areas that could bolster his administration’s legacy.

One key area is Africa, where the administration has won praise from members of both parties and the international community for increasing aid for economic development and to combat malaria, HIV/AIDS, and other diseases.

Fresh off a tour through Mozambique, Mali, Zambia, and Senegal, Mrs. Bush is urging Congress to reauthorize and give double funding to an emergency AIDS relief program that assists 120 countries worldwide, including 15 nations in Africa.

In a speech yesterday at the National Press Club that sought to confront the question, “Why Africa?,” the first lady made the case for sustained American aid, saying the nation had both a national interest and “a moral obligation to help” the continent’s many impoverished countries build stronger economies and fight the scourge of disease.

For Mrs. Bush, the Africa effort was her second recent foray into foreign affairs.

Last month, she penned an essay in the Wall Street Journal criticizing the regime in Burma for human-rights abuses.

It comes at a time when her popularity has remained high domestically and internationally even as her husband’s approval ratings, along with those of Vice President Cheney, have sunk to near-record lows.

“I can’t think of a better advocate than the first lady,” the coordinator of the president’s malaria initiative, Rear Admiral R. Timothy Ziemer, said in an interview yesterday. He said significant change was impossible without a “credible advocate” and that Mrs. Bush “enjoys huge receptivity” abroad. With Mr. Bush’s attention, at least internationally, squarely focused on Iraq and the broader war on terrorism, the White House is using Mrs. Bush as a quasi-ambassador to Africa, signaling that although the president may be focused elsewhere, the region remains a high priority in the administration’s final 18 months.

Having increased foreign aid for public health substantially, the administration sees an opportunity to leave a lasting mark, particularly in dealing with malaria and HIV/AIDS, Admiral Ziemer said. In her speech yesterday, Mrs. Bush tried at once to project a sense of optimism without understating the problems faced by African countries that are wracked by disease, poverty, and malnourishment. While calling the challenge of the AIDS pandemic “immense,” Mrs. Bush said American partnerships with African nations “are yielding progress” economically, citing advancements in education and the construction of more 1 million people in sub-Saharan Africa, up from 50,000 in 2003. She described meeting with people infected with AIDS who described the hope that comes with what is known as the “Lazarus effect” upon starting drug treatment programs. The effect, Mrs. Bush said, is that “people who once waited quietly for death now celebrate a second chance at life.”

“If you have the opportunity to meet these people, your question will no longer be ‘Why Africa?’ It will be, ‘Why not?'” the first lady said in conclusion.

In Congress, increased AIDS funding has bipartisan support, but a battle may loom over how the money is spent. Democrats oppose requirements that one-third of the money be directed toward initiatives that promote abstinence-only education, a spokeswoman for the House Foreign Affairs Committee, Lynne Weil, said yesterday.

The first lady did not wade deeply into dispute yesterday, noting only that the funding model “was developed by Africans.”

The first lady touted its success, saying it has helped provide antiretroviral treatment to more than roads, airports, hospitals, and community centers.

The first lady focused in particular on American initiatives to combat malaria and HIV/AIDS. A presidential program on malaria began in 2005 and provides medicine, insecticide sprays, and mosquito nets to eight African countries, where public health spending on the disease reaches as high as 40%.

Yet the administration’s more pressing concern is the reauthorization of the president’s emergency plan for AIDS relief, which began in 2003 as a five-year program providing $15 billion to countries around the world. The initiative expires next year, and Mr. Bush in May proposed doubling the program to $30 billion over another five years.