RICHMOND — Virginia regulators have voted to join a regional carbon cap-and-trade program, becoming the first Southern state to do so. The effort could lead to a 30 percent reduction in carbon emissions from the state’s largest power plants.

But there’s a hitch.

Republican lawmakers put language into the state budget that effectively prevents Virginia from participating.

Gov. Ralph Northam (D) could veto the language but must do so by a May 3 deadline.

Several environmental groups, which have supported Northam on conservation issues but have clashed with him on energy policy, turned up the pressure on him to act.

“Gov. Northam must now demonstrate leadership by vetoing the General Assembly’s procedural roadblock of the plan,” Walton Shepherd, Virginia policy director for the Natural Resources Defense Council, said via email.

Northam’s office declined to say what he intends to do.

“The governor applauds the board’s action and is in the process of reviewing pending legislation, including the budget. He has until May 3rd to take final action,” Northam spokeswoman Ofirah Yheskel said in a written statement.

Friday’s 5-to-2 vote by the State Air Pollution Control Board would put Virginia into the Regional Greenhouse Gas Initiative, or RGGI, a cap-and-trade program among nine other states in the northeast, including Maryland.

It puts limits on carbon emissions from the biggest power generators and sets up the trading of allowances on a market. Emitters that come below the cap can sell allowances to those who exceed it.

Republicans in the General Assembly argued this year that the system amounts to a tax on businesses. Del. Charles D. Poindexter (R-Franklin) sponsored a bill that prohibited the governor from entering into the regional program and gave authority for any such decision to the General Assembly.

That measure passed the legislature, but Northam vetoed it. He has said he supports the regional cap-and-trade initiative.

At the same time, though, Republicans put language into the budget that prohibits using any funds to join RGGI. Environmentalists argue that the governor has line-item veto power to strip the language out of the budget. But if he fails to act by May 3, the General Assembly would have to take up the matter in next year’s session.

On Friday, the air board structured its action so that it would go into effect as soon as the budget language is eliminated.

“Today’s vote is historic and takes us in the right direction in the fight to address climate change, protect public health, and to accelerate our transition to a clean energy economy,” Michael Town, executive director of the Virginia League of Conservation Voters, said via email. His group has been one of Northam’s top donors but has clashed with him over his support of a pair of natural gas pipelines in the western part of the state.

Northam is politically wounded after a scandal that emerged in February, when a racist photo from his 1984 medical school yearbook came to light. He apologized for the picture, which showed one person in blackface and another in Ku Klux Klan robes, then disavowed it — but admitted wearing blackface for a dance contest that same year.

Defying calls to resign, Northam promised to spend the rest of his term pursuing racial equity. Environmental groups immediately urged him to back that up by intervening to prevent a natural gas pumping station from being built in a historic African American community in Buckingham County, but he took no action.

Town made clear Friday that his group expects results on the carbon initiative.

“Virginians elected Northam to do the right thing for clean air and climate action,” Town said. “He has that opportunity in front of him and we expect him to follow through.”