Why did so many people decide not to vote for Hillary Clinton?

A theory from the 1950s can help explain.

According to University of Michigan researchers in their book The American Voter, just three factors influence the majority of voters – long-term partisan predispositions, judgments about important issues and images of the candidates.

The issues that matter most to the vast majority of voters are not “position” issues, the sort of issues that deeply divide the public, such as abortion and LGBT rights. Rather, voters decide based on “valence” issues, such as vigorous economic growth, the provision of high-quality health care, affordable educational opportunities, national security, and protection from terrorists and common criminals. Virtually everyone agrees that these are desirable ends of public policy, and political debate focuses on how to best achieve these goals.

Over the past several decades, this “performance politics” theory has been tested repeatedly and has held up remarkably well. High-quality academic survey data is not yet available for this year’s presidential election, but the results of hundreds of opinion polls tell us a great deal about how results aligned with this decades-old political theory.

1. A large group of voters didn’t identify with either party. The Democrats had a significant edge in party identifications over Republicans; 32 percent of people polled by Gallup in September said they identified Democratic and 27 percent said they were Republicans. But 40 percent said they were independents. With a large group without psychological ties to either major party, short-term forces evolving during the campaign – evaluations of party performance on key issues and candidate images – were crucial.

2. A number of important issues were not in Clinton’s favor. Nearly eight years after Barack Obama walked into the Oval Office, the economy’s performance remained problematic. Although unemployment had dropped sharply, growth remained anemic and the real incomes of millions of Americans were lower than they had been decades earlier. In addition, the national debt was at a historic high and increasing. Donald Trump charged that many of these economic maladies were caused by the failure of the Obama, George Bush and Bill Clinton administrations to make trade deals that would protect the American economy from unfair foreign competition. Simply put, the political-economic elites had sold out American workers.

Obama’s signature domestic legislation, the Affordable Care Act, was in trouble, with insurers bailing and premiums skyrocketing. And the situation wasn’t much better overseas, with much of the Middle East in turmoil. And Trump’s plan to build a wall between the U.S. and Mexico made immigration a high-profile and bitterly contentious issue. Trump charged that the Obama administration’s failure to control immigration posed serious economic, security and cultural threats.

Hillary Clinton’s basic difficulty with issues was that she was caught between Barack and a hard place. She could not risk openly criticizing the performance of Obama by proposing innovative solutions that might capture the public imagination. She desperately needed the president’s support on the campaign trail to help her mobilize African Americans and millennials, two groups that had been crucial for his electoral success in 2008 and 2012. Moreover, Clinton’s difficulties with separating herself from Obama on foreign policy issues were greatly magnified because she had been secretary of state when many of them were developed. She had no choice but to allow herself to be portrayed as an agent of continuity, not change.

The trouble for Clinton was that large numbers of people concentrated in states that were critical for her victory judged that performance very negatively. Many white working-class voters in Rust Belt states such as Michigan, Ohio, Pennsylvania and Wisconsin saw themselves as “forgotten Americans” who had been decidedly ill-served by Obama’s policy performance. Historically, these people had been a vital core support group for the Democratic Party. The Michigan researchers argued in their book The American Voter long ago that alienating these voters would be a recipe for electoral disaster.

3. Both candidates had very negative images. The Michigan researchers argued that candidate images are vital cues voters use to help them decide how candidates would perform in office. Trump was portrayed as a crass vulgarian whose volatile temperament and lack of experience disqualified him for major office. He countered by labeling Clinton “crooked Hillary,” arguing that her use of a private email system while secretary of state was criminal conduct that put America’s national security at grave risk. Unlike Bill Clinton, Hillary Clinton also had difficulty connecting with ordinary people, and polls indicate that many voters neither trusted nor liked her.

It is ironic that Clinton’s campaign ended with Obama stumping for her the day before the election in Ann Arbor, Mich., where The American Voter had been written six decades ago. Clinton was judged harshly by voters, especially a core constituency in her party, and she generated precious little enthusiasm across much of the electorate; the authors of The American Voter surely would have predicted there was a real risk she could lose.

(Harold Clarke is an Ashbel Smith professor of economics, political and policy sciences at the University of Texas at Dallas and the editor of the Journal of Electoral Studies. His email address is hclarke@utdallas.edu.)