Final answer:
Party unity votes peaked in 2010, indicating high partisanship and reduced bipartisan cooperation in the U.S. Congress. Since the 1990s, there has been significant polarization, with ideological extremes dominating and moderates fading away. This polarization has affected governance effectiveness and the passage of legislation.
Step-by-step explanation:
Party unity votes reached a 60-year high in 2010 in the United States Congress. The years after 1990 were characterized by a much greater degree of polarization than were the years prior to 1970. The year 1970 was marked by relatively weak partisanship.
During the 1960s and 70s, Congress saw higher levels of bipartisan cooperation and good relations across the political aisle. Moderate members of Congress were common, and legislation often had support from both parties. Bipartisanship was facilitated by regional differences within the parties that allowed for compromises on many issues.
In contrast, the past few decades have shown increased party polarization and a decline in effective governance. Fewer pieces of substantive legislation are being passed, and the challenge to presidential initiatives by the opposing party has become more prevalent. Divided government has also become more common, with voters often electing a president and a Congress from different parties.