Senate Republicans on Monday advanced a measure that would add Colorado to the list of so-called “right to work” states, prohibiting mandatory union membership.

Despite opposition from throngs of union members and leaders, as well as many in the business world, Republicans on the Senate Business, Labor and Technology Committee pushed the bill along on a party-line vote.

Senate Bill 55, sponsored by Sen. Tim Neville of Littleton, would prohibit an employer from requiring someone to join an organized labor group or pay dues as a condition of employment. It also would create civil and criminal penalties for violations.

The effort is familiar in the legislature, where several previous attempts to have Colorado join 28 other states with right to work laws failed.

In essence, the bill would ban closed union shops, in which an employer agrees to hire union members only, and employees are required to remain union members at all times.

“This is not an anti-union bill as some have said. It’s just the opposite,” Neville said. “This is a bill that says people have the right to choose.”

While Republicans have framed the measure as being pro-business, many business leaders fear that the effort would actually negatively impact the state’s economy and businesses.

“Our unique model, which is neither pro-union or right to work, has created a labor climate in Colorado that’s produced one of the nation’s most robust economies,” said Ron Tilton, president of FirstBank, who spoke on behalf of the Denver Metro Chamber of Commerce and other business groups.

Some in the business community, however, have split, including the National Federation of Independent Business.

“I’ve always believed no employee should be forced to join a union as a condition of employment, but should certainly be able to join a union if they choose,” said Mike Donahue, representing NFIB. “Right to work would be good not only for Colorado employees but would be a boost for the Colorado economy.”

But Leonard Martinez, a 39-year member of Sheet Metal Workers Local 9, said a right to work state would mean less training, which would set Colorado back in terms of economic production.

“What Colorado is poised to do … we have to be able to provide a workforce that brings our jobs on time, on budget,” he said.