More than 200 House Democrats issued a new warning Thursday as part of the contentious debate over infrastructure spending: Include strong union and labor protections or risk losing some of their support.

The message from nearly every member of the chamber’s political parties served as a new political marker as tense negotiations over potentially trillions of dollars in new public works spending continue. Senate Republicans are expected to make their own demands later Thursday morning, in the form of a new counter-offer in their ongoing talks with President Biden.

House Democrats emphasized in their letter that any new federal loans, grants, or tax breaks to improve the country’s infrastructure must be accompanied by a series of policy mandates to help workers. Companies that stand to benefit from this potential influx of government aid must make it easy for employees to unionize, pay prevailing wages, take action to prevent wage theft, and train workers for future positions through apprenticeship programs, according to lawmakers.

House Democrats expressed particular concern about the emerging clean-energy industry, which could benefit from billions of dollars in tax breaks and other new federal investments under Biden’s blueprint, known as the American Jobs Plan. According to lawmakers, the industry already has some of the worst worker protections in the United States, which they hope to improve as part of an infrastructure overhaul.

The effort was organized by three top Democratic lawmakers who represent the entire political spectrum: Rep. Pramila Jayapal (D-Wash.), leader of the Progressive Caucus; Rep. Stephanie Murphy (D-Fla.), co-chair of the fiscally conservative Blue Dog Coalition; and Rep. Susan Wild (D-Pa.), who chairs a key clean-energy task force with the moderate New Democrat Coalition.

The letter, sent to House Speaker Nancy Pelosi (D-Calif.), stopped short of explicitly promising to vote against an infrastructure bill if these and other labor-friendly provisions were not included. However, some Democrats involved in its creation said Thursday that any attempt to ignore their calls to action could cost them votes, posing new challenges for Pelosi and Biden given the party’s slim House majority.

Republicans, for their part, are likely to oppose any attempt to advance union-friendly policies as part of a larger package to improve the nation’s roads, pipes, ports, and Internet connections. A fight over labor policy in the coming months could complicate the prospects for a broader bipartisan agreement, especially since such a compromise already appears out of reach.

For the time being, talks between Biden and Senate Republicans continue, with only days until a self-imposed Memorial Day deadline by which the White House has said it expects to see progress — or it could try to advance infrastructure legislation with only Democratic votes. After shepherding his $1.9 trillion coronavirus relief plan through Congress with no GOP support, the president insists on a bipartisan solution.

Republican leaders are set to present their latest counter-offer on Thursday, after both sides presented updated infrastructure proposals last week that barely differed from their initial visions. Republicans are expected to call for roughly $1 trillion in spending while remaining opposed to the policy scope — and funding mechanisms — of Biden’s plan.

Some of the issues may already be addressed by state or federal law. Other aspects of Biden’s commitment are similar in approach or spirit to the PRO Act, which House lawmakers passed in February. The bill proposes significant changes to federal labor laws in order to assist Americans in forming or joining unions — and to protect them from potential retaliation from their employers. During his first address to a joint session of Congress this year, Biden urged Congress to adopt it.

However, the fight over the PRO Act may foreshadow some of the battles that the president and his Democratic House allies will face as they seek strong union and labor protections in the infrastructure package. The bill passed mostly along party lines in the House, only to die in the Senate, where Democrats have been unable to advance it in large part due to opposition from some of the party’s centrist members, not to mention Republicans.