KANDAHAR, Afghanistan — Ahmed Wali Karzai, the most powerful man in southern Afghanistan, may maintain links with drug dealers and insurgents, as some American officials and Afghans believe. And he might have played a central role in last summer’s fraudulent presidential election, as Western diplomats charged.

But Mr. Karzai is also the brother of the Afghan president, Hamid. And after debating Ahmed Wali’s future for months — and with a huge military operation in the area looming — Afghan and American officials have decided that the president’s brother will be allowed to stay in place.

Over the last several months, President Karzai has turned down repeated requests by both the American ambassador and the top American commander to move Ahmed Wali Karzai out of Kandahar, American officials here said. Many Western and Afghan officials say he stands in the way of building a just and efficient Afghan government, which they see as essential to dislodging the Taliban and eventually allowing American troops to withdraw.

Senior American officials spent months weighing the allegations against Ahmed Wali Karzai: that he pays off Taliban insurgents, that he launders money, that he seizes land, that he reaps enormous profits by facilitating the shipment of opium through the area. And the officials concluded that the evidence, some compelling, some circumstantial, was not clear enough to persuade the president to move his brother out of town, two NATO officials said.