The Service Employees International Union poured more than $23 million into its campaign to raise the federal minimum wage last year.

The money primarily went to nonprofit groups and public relations firms with the aim of making the effort look like an organic, grassroots movement.

That's up 35 percent from an estimated $17 million the previous year, according to the union's annual Labor Department financial reports.

"It has been increasing dramatically every year," said Michael Saltsman, executive director of the conservative Employment Policy Institute, which tracks the numbers. The union boosted its spending in Chicago alone by more than $1 million, he noted.

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