The Treasury Dept. has taken steps to implement a 2014 law that directed major changes for multiemployer pension plans. But some in Congress want to reverse a provision that lets sponsors of ailing plans seek to cut members' benefits.

Multiemployer plans cover about 10 million workers and retirees and are common in unionized construction. In 2010, coverage in the construction industry accounted for 55% of all multiemployer plans and 37.5% of the people those plans covered, the federal Pension Benefit Guaranty Corp. said in a 2013 report.