Political parties are linkage institutions, meaning they help connect people to the government. So, political parties have to perform five important tasks to be considered an effective linkage institution.
Parties endorse a candidate for office, and that's called a nomination. Very few people get elected to a public office without being nominated. For example, at the 2012 Republic National Convention, Mitt Romney was nominated over Ron Paul.
Parties coordinate campaigns by organizing events and publicizing their candidates.
Give Cues To Voters
Parties help people pick their candidates. For example, if you're a Republican and you see two candidates, a Republican and a Democrat, without knowing about either, you would probably pick the Republican. The voter assumes that Republicans support a general set of ideas, so it helps guide the voter.
Political parties advocate specific policy alternatives. For example, for many years, Democrats have supported abortion, while Republicans have been against it.
Parties help unify part of Congress. Whenever a Congressman wants to pass a bill, they would first consult with people of their own party for support. Parties are usually unified on a stance, which influences policymaking.