Interest groups are often criticized for being "single issue." This means they will support whichever political candidate promises to champion their one issue. The political candidate could have a terrible platform regarding every other issue, but as long as he or she supports the issue the interest group is promoting, the candidate will get the group's money and votes.

To use a non-partisan example, a powerful, well-funded interest group might decide they will only support a candidate who agrees to ban green hair dye. The honest candidates might say, well, that is completely ridiculous, and I will not support that, because I stand for creating jobs, cleaning up corruption, etc. The interest group, however, could easily use its money to defeat these sensible candidates. The person who got elected might destroy jobs, be corrupt, and not even believe in democracy, but he or she would get elected because of supporting the hair dye ban. He might even do grave damage to the economic interests of the people who voted him into office. 

Many therefore say that groups should support candidates not on the basis of single issues but based on a broad range of issues that might work together to make the world a better place. 

There are two main criticisms of interest groups in the United States’ political system.

First, there is the criticism that they, in a sense, corrupt officials.  Interest groups are constantly lobbying elected officials.  They donate money to their campaigns and to their PACs.  Many people worry that the donations allow the interest groups to effectively “buy” the politicians.  Even if the politicians are not consciously voting in certain ways based on who donates money to them, they can be influenced by the money.  They might only talk to interest groups who donate money to them, which would make it so they only hear the viewpoints of those interest groups.

Second, there is the criticism that interest groups allow the rich to have more power than they should.  From this point of view, the successful interest groups are the ones that have a lot of money.  They use the money for contributions, but also to pay lobbyists and to mount public relations campaigns.  This means that interest groups that have money can have a “louder” voice than interest groups that have less money, even if the poorer interest groups have more members.

Thus, people sometimes criticize interest groups for corrupting public officials and/or for giving the rich more power than they should have based on their numbers.