WASHINGTON  As President Obama prepares to decide whether to send additional troops to Afghanistan, the political climate appears increasingly challenging for him, leaving him in the awkward position of relying on the Republican Party, and not his own, for support.

The simple political narrative of the Afghanistan war  that this was the good war, in which the United States would hunt down the perpetrators of the Sept. 11 attacks  has faded over time, with popular support ebbing, American casualties rising and confidence in the Afghan government declining. In addition, Afghanistan’s disputed election, and the attendant fraud charges that have been lodged against President Hamid Karzai, are contributing further to the erosion of public support.

A CBS News poll released on Tuesday reports that 41 percent of those polled wanted troop levels in Afghanistan decreased, compared with 33 percent in April. Far fewer people  25 percent  wanted troop levels increased, compared with 39 percent in April. And Mr. Obama’s approval rating for his handling of Afghanistan has dropped eight points since April, to 48 percent.

Congressional Democrats, particularly those on the left, report increasing disenchantment among constituents with the idea of a long and possibly escalating conflict in Afghanistan, especially as the American strategy comes to resemble a long-term nation-building approach rather than a counterterrorism operation.