Let's take, for example, a group of people who are in favour of paid medical leave. Without a way of organizing, they have no chance of making it come true. Political parties give them a way of voicing their opinion - this is an example of how how
political parties strengthen U.S. democracy.
An example of weakening is that the parties spend a significant portion of their time fundraising and can therefore be influenced by rich lobbyists -which is not democratic (it's not democratic that the few rich have more power than the poor)