Okay, first of all, passing legislation that commits the United States to follow goals set out by the United Nations is not ceding power to the U.N. It is democratically voting to follow measures determined by the U.N. If the goals determined by the U.N. were determined to be not in our best interest then the US could change it. To call this a "global tax on the United States" is unfair and intellectually dishonest.
Second, the millennium development goals state as their number one priority the elimination of EXTREME poverty. Jeffrey Sach's also outlines this in his book "Thje End of Poverty" and argues that while the complete elimination of poverty may not be possible the elimination of extreme poverty - living on less than $1 a day - is very possible and can happen in our lifetimes.
It is a plain fact to me that the world has become smaller. There are no longer oceans separating the globe's citizens; there is no separation. Turn on CNN, open a national Geographic or search google and there's the world, right there in front of your face. There are your neighbors. And as this world becomes increasingly small there is mind numbing growth in the distance between rich and poor. If you are lucky enough to be born in a developed nation, then you have unbelievable access to opportunity, riches unprecedented in the history of civilization If you are born in other parts of the world, you might be greeted with Hunger...or AIDS, or mosquitoes spreading Malaria.
If your neighbor was hungry, would you bring them dinner? Who would say "no" to that? Every citizen on Earth is now our neighbor and we cannot say "no" to them either. Today, we have an opportunity and the resources to confront and eliminate global extreme poverty. As such, we have a moral imperative to do so. There should be no question about increasing our foreign aide.