President Bush and his Democratic challenger John Kerry have both described the killing in Sudan's Darfur region as "genocide" - but neither supports sending in U.S. troops to bring the situation under control.
The candidates outlined their positions during the first of three U.S. presidential debates.
Both men said they would work with the African Union as it tries to stop pro-government Arab militias that are accused of killing tens of thousands of black African civilians.
President Bush, a Republican, promised humanitarian aid on top of more than $200 million already committed.
Senator Kerry went further, saying as president he would provide logistical support to African Union troops. But he said U.S. forces are overextended because of the war in Iraq.
Mr. Kerry also said the country and the world have a "moral responsibility" not to allow another Rwanda. In 1994, Rwandan Hutu extremists slaughtered 800,000 minority Tutsis and their Hutu sympathizers in a three-month killing spree.
President Bush said earlier this year that the United States is "appalled" by the violence in Darfur, and said it is clear that only "outside action" can stop the killing.