(Reuters) - Opponents of Indiana's new "right to work" law have withdrawn a request for a court order to block the anti-union measure after the state said it will not enforce it retroactively.

But opponents of the law said on Monday they will press on with their larger legal challenge and may seek a preliminary injunction against the measure on other grounds in federal court in the coming weeks.

Last month, Indiana became the 23rd state to pass "right-to-work" legislation and the first in the nation's manufacturing heartland to do so, drawing a challenge from union members, who claim it violates a number of constitutional provisions, including the equal protection clause.

Attorneys for the International Union of Operating Engineers Local 150 asked U.S. District Judge Philip Simon in Hammond, Indiana to issue an emergency temporary restraining order (TRO) to block the law. Simon was scheduled to hear the request on Monday.

The union had been concerned Indiana intended to enforce the right-to-work law retroactively on collective bargaining agreements already negotiated.

But when the state of Indiana said that those fears were unfounded, the union decided to drop its request for the TRO.

In a filing with the court late last week, the state attorney general's office said, "The right-to-work law does not apply to existing contracts and . does not retroactively apply to actions before it became law."

Marc Poulos, one of the attorneys challenging the law, said while the government filing made an emergency TRO unnecessary, it did not derail the case against the measure.

"The fact that they interpret it that way eliminates those two arguments," Poulos said. "But that doesn't take away from the fact that we would likely follow up with a preliminary injunction on other grounds. That's probably the road we're going down."

Bryan Corbin, a spokesman for Indiana Attorney General Greg Zoeller, who is defending the law in federal court, agreed "the underlying issues in this lawsuit have not yet been litigated."

Supporters of Indiana's "right-to-work" measure, which bars union contracts from requiring non-union members to pay fees for representation, say it was needed to attract and retain businesses in the state.

Opponents claim it violates the equal protection clause of the U.S. Constitution by requiring dues-paying members to furnish free representation to people who refuse to join the union or pay dues.

Opponents say the measure is also designed to hobble organized labor, a key financial supporter of the Democratic Party, and to lower the wages and benefits of Indiana workers.

So far, efforts to overturn similar anti-union laws in other states have been unsuccessful. Patrick Semmens, a spokesman for the National Right to Work Legal Defense Foundation, which submitted a brief supporting the Indiana measure and has helped defend other states' "right-to-work" laws, predicted the Indiana challenge would fail.

"We're very confident that the law will be upheld," Semmens said.

The Indiana law, the first adopted since Oklahoma enacted a similar measure a decade ago, allows workers in unionized jobs to opt out of paying dues. Most "right to work" states are in the South and West.

Similar bills have been introduced recently in Maryland, Missouri, New Hampshire, New Jersey, and West Virginia, according to the National Conference of State Legislatures.

(Editing by Paul Thomasch)

(Photo via Shutterstock)