The death of Senator Robert Byrd on Monday has upended the administration’s cap and trade offensive, as observers of West Virginia politics suspect his yet-to-be-named interim successor will defer to the state’s pro-coal politics on pending climate legislation.

Byrd, whose unprecedented fifty-one-year senate career bore out a series of outright and often stunning political evolutions, had warmed in recent years to overhauling the nation’s climate regulations despite broad opposition from his home state. His death, and the loss of a reliably-Democratic vote, has forced Senate leadership to reassess the viability of the White House’s aggressive climate legislation push in this difficult election year for Democrats.

“It is a tougher road, believe me,” Senator Dick Durban, the Democrats’ chief vote whip, said Monday. “A 58-vote majority is not as good as a 59-vote majority.”

Wildly popular and the leading candidate for the seat in 2012, West Virginia’s Democratic Governor Joe Manchin has not yet floated the names of potential replacements, saying only that he will not appoint himself. Instead, Manchin will tap an ally whose politics–and posture on coal, the state’s bread and butter issue–mirrors his own.

Though among the first governors to endorse then-Senator Barack Obama’s presidential bid, Manchin has been in open rebellion against the Environmental Protection Agency and the president’s legislative agenda to reconstruct the environmental regulatory regime. It’s his rebellious streak–a level of insubordinance on climate issues that has further endeared Manchin to voters in an otherwise hostile cycle–that Capitol Hill Democrats worry may derail the fragile cap and trade coalition they have assembled.

Democratic operatives and environmental lobbyists are concerned that Manchin will employ a “pro-coal litmus test” in selecting Sen. Byrd’s replacement.

“It would be unfortunate for the state and the country if Gov. Manchin has a litmus test that anyone who replaces Sen. Byrd must side with the coal industry and against the people of the state and country,” one lobbyist told the Charleston Gazette.

Manchin, who declared last year coal to be West Virginia’s official state rock, likely has a different interpretation of the litmus test question as he mulls his near-certain 2012 senate campaign.

“Manchin must maintain his present course, one of outright rebellion against the White House, if he wishes to survive the Obama backlash,” a keen observer of West Virginia politics noted. “Anything less in his appointment decision will be seen by voters as caving to Washington.”