Pennsylvania Senate Republicans have told Gov. Tom Wolf they will block new appointments to the state utility commission unless he backs down from his effort to join a multistate climate plan to cut power-plant emissions.

The Republicans, who object to the Democratic governor’s move to join the Regional Greenhouse Gas Initiative (RGGI), said in a letter to Wolf on Wednesday that they will block his nominations to the Pennsylvania Public Utility Commission until he withdraws his executive order related to the climate initiative.

“We do not take this position lightly,” said the letter signed by all 27 Republican senators and an Independent state senator from Luzerne County who caucuses with the Republicans.

The move is significant because two seats held by Democrats on the five-member commission have expired, and, without new appointments, control of the PUC will flip to a 2-1 Republican majority soon. The commission is independent. Though it operates more like a judicial body and is not overtly political, the PUC has significant influence over energy policy.

Lyndsay Kensinger, the governor’s spokesperson, denounced the Republican effort to hold PUC appointments “hostage” as a “reckless precedent,” and said Wolf will not pull back on his efforts to enter the Greenhouse Gas Initiative.

“It is appalling that during Earth Week the GOP is choosing to play politics not just with the PUC, an independent commission, but also with climate change at a time that they have suggested zero solutions and have actively opposed any meaningful action,” Kensinger said in an email.

Wolf in 2019 signed an executive order to join the regional market-based program to reduce greenhouse gases from electric generation, a system that rewards power plants that emit little or no carbon dioxide and penalizes fossil-fuel generators. Eleven other states have joined RGGI — Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Virginia.

The plan would impose limits on greenhouse gas emissions that tighten each year, requiring power plants that emit carbon dioxide to pay increasingly higher prices for emissions allowances. Republicans object to such a regional cap-and-trade system, saying it amounts to a $500 million carbon tax on Pennsylvanians.

The GOP also cites studies that predict there will be little reduction in overall emissions. The argument is that power generation will move to more competitive fossil-fuel plants in other states not affected by the emissions caps, a migration of emissions known as “leakage.”

They say Wolf should get the consent of the General Assembly to join any multistate agreement. Wolf has vetoed legislation that would force a legislative vote on joining the initiative.

A political impasse over PUC appointments is not unprecedented. The commission operated with as few as two members in the 1980s when Republican Gov. Dick Thornburgh and the Democratic General Assembly were locked in a standoff. The PUC legally can function with as few as one commissioner. Commissioners are appointed to five-year terms, one of which expires each year.

The PUC’s current vacancies began last year when Commissioner Andrew Place, a Democrat, resigned at the end of his term. Wolf nominated Hayley Brook, a senior adviser on energy and climate at the Department of Environmental Protection who helped craft the administration’s climate policy, to fill the vacancy in August. The Senate did not take action during last year’s session, and Wolf resubmitted her name in January.

A second five-year term, now held by PUC Vice Chairman David W. Sweet, a Democrat and adviser to Wolf and former Gov. Ed Rendell, expired on April 1. Wolf has not reappointed Sweet or nominated a replacement. According to state law, Sweet can remain in the seat for up to six months after his term expires if a successor has not been duly appointed.

Even if the Republicans assume a 2-1 majority on the PUC with the two vacancies, the chairmanship would remain with Gladys Brown Dutrieuille, a Democrat who has served on the PUC since 2013. Under state law, the governor gets to name the commission’s chair.