In actuality, those who run in either of the two parties are for the most part out for their own power and money. But in terms of policy statements, the democrats do tend to take into account those of lower incomes and realize that we need a middle class in order to survive as a nation.
The political parties did not really gain distinction among the American populace until the industrial revolution. The Republicans were backing big industry against the regulation of labor. Minimum Wage and limited hours were among the issues that industry was fighting. The democrats believed that these were things necassary to keep a decent quality of life for the American people. Unions were formed and negotiations began for labor contracts from he efforts of the Democratic party. The rich want to keep their wealth, so they elect people that believe lower taxes and less money required for responsibility to the people. Democrats believe that industry and corporations will do whatever it takes to make more profit, and that includes abuse of the working class. This was Marx's prediction as to the fall of capitalism.
Yes, when it comes to mainstream parties being better for the poor and low income in America, there isn't even a viable debate that could be made that the Republicans would be the better of the two parties for the poor. The Republicans go out of their way to cut funding for programs that help the poor. The Democrats definitely try to pass more laws with the poor in mind.
I do believe that the Democratic party is better for lower-income Americans because it provides more services to them. The Democratic party supports things like welfare, socialized medicine, and many other programs that benefit those with lower incomes. They also support things like more taxes for the rich, which would help fund social programs.
Think of it like two parents. Whenever the child needs money, the mother is happy to give to her kid. No matter what he's done or what he deserves, he gets equal money. Now picture the father only giving money to the child if he is to work for it and earn it himself. Of course the mother will be more liked temporarily, but in the long term, the father (of course representing the Republicans) will give the child a sense of discipline and work ethic so he is no longer financially dependent. It isn't about comforting the lower class, its about getting them out of the lower class.
The Democratic Parties focus is on others as a whole and the Republican party focuses on the individual and self. The Democratic party is about the betterment of mankind where Republicans have a focus of the strongest survive and feel they should not be required to help anyone whereas the Democrats feel it is there civic duty.