The main similarity between political parties and interest groups is that they both seek to achieve certain policy objectives. These can be a broad range of policy objectives or only a relative few.
Political parties have a large number of policy objectives that they wish to achieve, whereas pressure groups tend to have only a few. Some pressure groups, such as those that campaign for the protection of the environment, are based around a single issue.
As such, pressure groups tend to have greater coherence than political parties, as it is much easier for their members to unite around a common single objective than a broad range of policies.
This helps to explain why pressure groups endeavor to bring about changes in policy without attaining political power. Their focus is so narrow that it would be virtually impossible for them to secure the kind of broad-based coalition that is essential in a democracy for a political party.
However, despite remaining outside of the formal democratic process, pressure groups—as their name implies—can still exert considerable pressure on policy-makers in order to get the changes that they seek.
Such changes are not always forthcoming, however, because political parties tend to be quite broad-based coalitions. This means that policy-makers need to take into consideration a broad range of stakeholders whose interests are often opposed to those of relevant pressure groups. As a consequence, any changes made by political parties in power tend not to be as bold or as radical as pressure groups, who don't have to deal with the necessary compromises of political power in a democracy, would like.