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Data: Bureau of Labor Statistics; Map: Lazaro Gamio/Axios

AFL-CIO President Richard Trumka has challenged 2020 Democratic candidates to prove their support and not to take the union vote for granted — but there are half as many union members today than there were just 35 years ago, according to the Pew Research Center.

Why it matters: Half of Americans say the decline of unions has had a negative impact on workers, according to a Pew Research Center survey. And it may be a central reason for stagnant wages and growing income inequality in the U.S., according to a new study by the Brookings Institution's Hamilton Project.

Between the lines: President Trump's appeal to blue-collar workers was a major factor in his victories in what were once big union states such as Michigan, Wisconsin and Pennsylvania. Hillary Clinton won the union vote by the narrowest margin in decades.

  • But the decline hasn't made labor a less important part of the Democratic coalition, as top 2020 candidates such as Joe Biden, Bernie Sanders and Elizabeth Warren have already begun to woo union voters.
  • Our thought bubble, from Axios' Dan Primack: Sanders and Warren have been unabashedly pro-union. An argument could be made that one of them becoming president could be the one thing that could bring back unions. It would be a central part of their campaign — not a "take for granted" sidelight.

By the numbers:

  • Despite low wages and severe income inequality, the percentage of U.S. workers who are union members has fallen from 20.1% in 1983 to just 10.5% in 2018, according to Pew.
  • Dying union membership has been most pronounced in states such as Indiana, Michigan, Ohio, Pennsylvania and Wisconsin — states where manufacturing employment has plummeted and Trump won in 2016, per Brookings.

The main reasons for the decline, according to Brookings:

  1. The shift from manufacturing to a services-based economy.
  2. More people are getting college degrees, and workers with a high school degree or less have typically been more likely to have union jobs.
  3. The rise of tech, with the attraction of more high-paying, nonunion jobs for the highest-skilled workers.
  4. Deregulation, which made it easier for nonunion employers to compete.
  5. The spread of right-to-work legislation, which allows for some workers to receive the benefits of unions without paying dues.
  6. Aggressive employers who have used tactics like delaying union elections, hiring consultants to help fight unionization and publicly opposing unions.

What to watch: Public sector unions have maintained their strength over the past several decades, according to Brookings. But a recent Supreme Court decision preventing public sector unions from collecting fees from nonunion employees could have a significant impact on membership.

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