Labor unions contributed more than $1.6 billion to left-wing political advocacy groups between 2010 and 2018, according to a new report.

Organized labor's political spending stands in stark contrast with how membership votes. While 2016 exit polls showed that nearly half of union households voted Republican, 99 percent of all union political contributions since 2010 went to Democratic causes. Unions gave more than $1.6 billion to Democratic political operations and liberal advocacy groups, according to a Center for Union Facts analysis.

Major recipients outlined in a new watchdog group report include the Democratic Governors Association, the Center for American Progress, and Planned Parenthood.

Rick Berman, executive director of the Center for Union Facts, said many workers are unaware that a major portion of their union dues and fees are used to finance political operations they might otherwise not support.

"Many union members have no clue that their dues are being diverted to groups that have no impact on collective bargaining issues," Berman said in a statement.

Of the $1.6 billion in union political spending, more than $458 million went to groups aligned with the Democratic Party, including the Clinton Foundation, the National Democratic Club, and Planned Parenthood. More than $384 million went to groups that focus on advancing progressive economic causes. Nearly $11.5 million went to left-wing media operations.

Big Labor's political activities played a major role in the 2018 Supreme Court case Janus v. American Federation of State, County, and Municipal Employees. Several government employees sued to end mandatory union dues and fees because they amounted to compelled speech. The Court declared forced dues schemes unconstitutional in a 5-4 opinion, but the ruling only applies to government workers.

Berman said lawmakers should seek similar protections for private sector union members through the Employee Rights Act. The bill requires written consent for an employee's dues to be used for any purpose "not directly related to the organization's collective bargaining." Berman called the bill "common sense."

"Common sense legislation like the Employee Rights Act would restore transparency to today's labor movement and ensure private-sector workers are no longer compelled to support political advocacy that they don't agree with," he said.

Hillary Clinton won union voters in 2016 by the narrowest margin of any Democrat since 1984. Democrats vying for the 2020 nomination have gone to great lengths to woo organized labor. The substantial financial and political resources possessed by union leaders means 2020 Democratic presidential candidates are "falling all over themselves" attempting to win support.

Sen. Bernie Sanders (I., Vt.) proposed a plan to double union membership just days after touting the endorsement of a union that has embraced the anti-Israel BDS movement. Former vice president Joe Biden labeled himself a "union man, period" at his first campaign rally. Sen. Elizabeth Warren (D., Mass.) and South Bend mayor Pete Buttigieg have both appeared in union picket lines and rallies in recent months.

The Democratic scramble to win the union vote comes at a time of century-low union membership. Only 10.5 percent of American workers are in unions, according to the Bureau of Labor Statistics.