While pressure groups aim at influencing government decisions, the primary aim of political parties is to

The primary aim of political parties is to control political power. Political parties compete for the support of voters and seek to win elections in order to control the government and implement their policies and programmes. While political parties may have specific policies that appeal to certain groups of people, their ultimate goal is to gain power and govern. This is different from pressure groups, which may also seek to influence government decisions, but do not necessarily seek to control political power.