Public opinion, political parties, and interest groups all play a role in the American political system. It is often a combination of these factors that lead to the decisions made by political leaders regarding policy and legislation.
Political parties typically have a certain platform, which is a particular set of goals that they aim to achieve. Political parties are formed by people who share similar beliefs or hold a similar set of goals that they hope to achieve. In American politics, a politician can occasionally break from some of the goals of their party, but in order to receive the backing and support of the party in elections they are typically expected to vote with the other members of their party regarding legislation, particularly on issues of importance to the party.
The role of public opinion is important in American politics because in the end it is the American people who elect their leadership. Public opinion, or what the public thinks of you, plays a big role in whether or not the constituents will vote for you. If a politician is mired in scandals, they will likely be surrounded by a negative public opinion. This can be capitalized upon by opponents come election time, sometimes with stunning results. A recent example of this can be seen in the US Senate Special Election for the State of Alabama between Roy Moore and Doug Jones. Alabama is traditionally a very conservative state. Due to the scandal surrounding Roy Moore, a Republican, regarding his alleged sexual misconduct decades earlier, the public opinion of him dropped significantly. This, combined with the general unpopularity of President Trump at the time, created the perfect opportunity for Democrat Doug Jones. The result was a victory for Doug Jones, as he became the first Democrat to be elected as a senator from the state of Alabama in 25 years. This is evidence that public opinion still holds a major role in American politics and can truly sway an election.
Interest groups also play a significant role in American politics. Interest groups can be made up of organizations and/or citizens who have a certain political goal or goals in mind. An example of an interest group in the United States would be the NRA. The NRA or National Rifle Association focuses their political goals around the 2nd Amendment to the US Constitution (the right to keep and bear arms). The NRA, and other interest groups, typically influence politicians through a process called lobbying. They attempt to get politicians to support their views through a number of means. One way an interest group might operate to influence a politician is running ads for them if they support them or against them if they oppose them. The NRA might do this by funding commercials highlighting a candidate's strong views supporting the second amendment in an election that is expected to be close. Their hope would be that members of the NRA or those who support their views would be more likely to vote for that candidate.
Interest groups may also help to fund a candidate's campaign though other means, such as holding rallies or distributing pamphlets and emails to their members and to the public. It is important to remember that the goal of an interest group is generally not to provide an equal view of both sides of their issue. Their goal is to promote their own beliefs and to make that option seem more appealing. It is also important to note that the goals and desires of special interest groups are typically going to benefit them and do not necessarily benefit the nation as a whole.
As you can see, American politics certainly have a number of factors that play into the decisions that are made and who is making those decisions. This is true of many other democracies around the world as well. It is important for politicians in a democracy to take into account public opinion, the views of their party, and the pressure from interest groups when crafting their campaign and goals. It is necessary that they maintain a balance between these aspects of politics, particularly when they come into conflict. Failure to acknowledge any one of these three areas can lead to the end of their political career.