I am perplexed.. Why does Pres. Bush, nancy Pelosi, Barbara Boxwer, etc want amnesty for the illegals?

There has to be a reason but I have NOT been able to find it. Amnesty for illegals will severly devastate the American economy to give social welfare, education, food stamps, medical, etc to 30 million illegals. There has to be another reason why Pres. Bush, John McCain, Harry Reid, and others do NOT want to close the border. Please, I am looking for any ideas as to why