The Poor are a voter group.
Poor people are very sensitive to poverty and unemployment, so they want the rates of both to be as low as possible. They're also the main recipients of welfare and government aid programs, so they support them when they can. They simultaneously oppose both taxes and tax breaks, since they already have so little money and usually aren't eligible to claim any breaks.
Poor people aren't predisposed to any other voter groups.
When middle earnings fall below average, middle income earners will begin falling into poverty, increasing the number of poor people. When poor earnings rise above average, poor people will begin rising out of poverty, decreasing their numbers.
Poor people are sympathized with many other groups, with socialists being the most important. Naturally, the poor often have disagreements with people who have more money than they do and aren't liked by capitalists at all.
Poor people don't have their own pressure group, but poor socialists are more likely to join the Communist Party, poor patriots are more likely to join the Patriot Party, poor environmentalists are more likely to join the Friends Of The Planet, poor ethnic minorities are more likely to join Equality Now!, and poor farmers are more likely to join the Farmers Society.