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Labor unions rely on density for two reasons. First, unions need monthly dues in order to be able to pay organizers and staff. The more members, the more dues, the more organizing. Second, the more unions that exist in a region, the easier it is to organize. That's because more union employers usually means smaller disparities between the wages paid by different businesses -- meaning less incentive to oppose organizing drives. That's why New York state has the highest union membership rate in the country: New York City's high union density makes it easier to organize.
Again, half of the country's union members work in government. That's high density. Public-sector employee unions work hard -- and spend heavily -- to support elected officials that support their organizing efforts.
And that's why Walker gives labor such conniptions. He demonstrated that unions could be beaten -- and that even in a state that twice backed Barack Obama, he didn't pay a political price for it. Walker has made his defeat of the unions a core part of his pitch to conservative voters. So far, he's running with the front of the pack."
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