Voters are not the only actors. Politicians, parties, donors and the rules of the game shape what choices citizens face. This level looks at how institutions and elite incentives may drive polarization, and at a long-running debate about whether one party has moved further than the other.

A surprising starting point

In 1950 a committee of the American Political Science Association issued a report titled Toward a More Responsible Two-Party System. Its complaint was that American parties were too similar and too internally mixed, so voters could not tell what they stood for or hold them responsible. The committee wanted clearer, more disciplined parties offering distinct programs. In a sense, the United States eventually got what the report asked for. This history is a useful reminder that some degree of party difference is healthy: it gives voters real choices and makes accountability possible.

Primaries

One common explanation points to party primaries. Because primary voters tend to be more engaged and more partisan than general election voters, candidates may fear a challenge from their own flank more than defeat by the other party. Reformers have tried open or "top-two" primaries, such as California's system adopted in 2010, to reward moderates. Evidence that these reforms reduce polarization has so far been modest and mixed. Part of the reason may be that primary turnout is low under any system.

Gerrymandering

Many people blame gerrymandering, the drawing of district lines to favor a party. It can certainly create safe seats and distort representation. But as a cause of polarization the evidence is weak. In a 2009 study, Nolan McCarty, Keith Poole and Howard Rosenthal noted that the Senate, whose members represent whole states with fixed borders, polarized almost as much as the House. Most of the difference between districts comes from where people live, not where lines are drawn.

Close competition

The political scientist Frances Lee, in Insecure Majorities (2016), offered a fresher institutional explanation. For most of the period from the 1930s to the 1980s, Democrats were the clear majority party in Congress and Republicans had little realistic hope of winning control. Since 1980, control of the Senate and House has changed hands often. Lee argued that when control is constantly up for grabs, each party has a strong incentive to deny the other any shared accomplishment and to sharpen differences before the next election. Cooperation that would help the opponent's record becomes costly. On this view, part of the hostility is not ideological at all but a rational response to intense competition.

Money and activists

Political donors and activists are, on average, more ideological than ordinary voters, and they supply candidates with money, volunteers and attention. Small online donations tend to flow most to candidates with national profiles built on confrontation. Researchers debate how large these effects are, but the direction of the incentive is not in doubt: fiery messages raise money.

The asymmetry debate

Does polarization in the United States come equally from both sides? Roll-call measures since the 1970s show congressional Republicans moving further from the center than congressional Democrats. The scholars Thomas Mann and Norman Ornstein, one from a center-left and one from a center-right think tank, argued in 2012 that the Republican Party had become an ideological outlier, scornful of compromise and dismissive of its opponents' legitimacy. Matt Grossmann and David Hopkins, in Asymmetric Politics (2016), offered a different kind of asymmetry: Republicans as a movement united by broad conservative principles, Democrats as a coalition of social groups seeking specific policies.

Others point to evidence running the other way on some issues. Survey data show that in the 2010s Democratic voters, especially white college-educated Democrats, moved substantially to the left on immigration, race and social questions, faster than Republican voters moved right. Critics of the asymmetry thesis also note that roll-call scales measure votes on bills that leaders choose, and may miss cultural shifts. A fair summary is that the answer depends on which measure, which issues and which period, and that each side tends to notice the other's movement more than its own.

Elites leading the public

Whichever way the asymmetry runs, a broad finding is that many voters take their cues from party leaders. When a party's leaders change position, many of its voters follow. This means elite polarization can produce mass polarization over time, and it also means leaders bear special responsibility for the tone of public life.

Worked example: the shared bill

Imagine a bipartisan bill to repair bridges, written by members of both parties. It would help voters in every district. Six months before an election in which control of the chamber is at stake, the majority party's leaders schedule a vote. Should the minority party support it?

If the minority votes yes, the bill passes with a bipartisan majority, and the majority party campaigns on "getting things done." If the minority votes no, it can argue that the majority is ineffective. Frances Lee's theory predicts that the closer the race for control, the more likely the minority is to oppose even a bill many of its members like. Notice that this outcome requires no ideological disagreement about bridges at all. It is the structure of competition, not the content of beliefs, doing the work. Reformers who focus only on persuading voters may miss incentives of this kind.