“The issue now is when do we start withdrawing troops and at what pace,” one senior administration official said. “Petraeus wants as much time as he can get,” the official said, but added that “the president may not have the leeway” to give him that time. The reality, officials said, is that starting around April the military will simply run out of troops to maintain the current effort. By then, officials said, Mr. Bush would either have to withdraw roughly one brigade a month, or extend the tours of troops now in Iraq and shorten their time back home before redeployment. The latter, said one White House official, “is not something the president wants to do” and would likely become a centerpiece of the 2008 presidential campaign.