Political parties influence public policy precisely because that is why they exist: to influence public policy. Political parties are large groupings of like-minded individuals who collectively seek to influence public policy through the election of candidates for political office from that party and through pressure placed on governing institutions for changes in or support of existing policies and practices.

Political parties are formed around shared convictions regarding the role of government in society and the perceived requirement for legal structures intended to protect favored policies or, conversely, to change those policies with which they disagree. In the United States, the two major political parties have fundamental disagreements on a wide range of policies, from reproductive and gun rights to the role of government in the provision of basic services like health care. There is considerable overlap in many of these issue areas, but generalizations can be made for the purposes of discussion. There are, for example, liberal Democrats who agree with most Republicans on the need for a strong national defense and there are Republicans who agree with liberal Democrats about the need to regulate some categories of firearms. There are some, called “Blue Dog Democrats,” who side with conservatives on issues of fiscal restraint and there are Republicans who vote with Democrats on the need to ensure access for women to reproductive health options. Ideological distinctions are generally obliterated when parochial considerations enter the equation. The most liberal Democrat will support weapons systems built in his or her congressional district or state and the most conservative Republican will support specific social welfare programs intended to help those most in need.

The role of political parties is largely the same around the world, with specifics adapted to unique circumstances. In Russia, the United Russia party represents the most virulently nationalistic of Russian voters and interests, while the Communist Party seeks restoration of the old political order with ideological roots in the economic and political theories of Karl Marx and Vladimir Lenin. In China, the only political party of consequence is the Communist Party, which retains through force of arms a monopoly on political power in that enormous country, although differences exist within the Communist Party on specific issues like economic modernization.

Political parties exist to influence public policy. They do so through slates of candidates for political office, through the raising of money to advance their political agendas, and through pressures applied on elected officials and on government agencies to conform to favored policy proposals. In a democratic system, including in a republic, political parties are enormously influential for the role they play in putting forth candidates for elective office and in helping successful candidates remain in office. They provide financial resources as well as personnel and run advertisements on behalf of candidates from their party. Through all of this, they influence public policy in the direction consistent with their ideological inclinations.

I think that policy making is impacted by political parties in a couple of ways.  The most basic element is that an elected representative who is loyal to their party must generate policies that reflect the overall platform of the party.  If not, they certainly must speak out against policies that go against party platform ideas.  In this, political party influence policy making because it forces representatives to make fundamental choices between what is advocated in the legislative sphere and what is embraced by the party.  It is essential for the representative to ensure that they have the support of the political party.  Losing this is almost as bad as losing the support of their represented people.  The political party influences the direction of policies advocated and initiated in that the legislation that comes out of the representative sphere of government is reflective of the party's belief system.  In the past summer, the United States political sphere saw this with the debate on the raising of the debt ceiling.  The Republican Party was committed to not raising the debt ceiling.  This became part of the party's belief system.  Members of the Tea Party wing of the Republican Party were committed to this idea.  In the end, this is a good reflection of how political parties can influence policy making.