Final answer:
Primary elections are a method used to select party nominees and can contribute to the rise of partisanship in U.S. politics. Political parties influence the primary system by determining whether it is open or closed. One potential reform is the adoption of open primaries.
Step-by-step explanation:
Primary elections are a method used to select party nominees for state, local, and presidential contests. They can have an impact on the rise of partisanship in U.S. politics because candidates who survive the primaries tend to align themselves with committed partisans and may have to moderate their image for the general election. Political parties influence the primary system by determining whether it is open or closed, with closed primaries preferred to ensure party members select the nominee. As for potential reforms, one proposal is the adoption of open primaries, which allow any voter to vote for any candidate regardless of their party affiliation.