Ken BuckRepublican Senate hopeful Ken Buck of Colorado — a climate skeptic — is likely to replace Sen. Michael Bennet, who supports climate legislation.Cross-posted from Wonk Room.

A Wonk Room analysis finds that there are six key Senate races in which a strong vote for climate action runs a serious risk of being replaced by a global-warming denier. Of the 37 U.S. Senate races this fall, 36 involve Republican candidates who are global-warming deniers or oppose climate action (Vermont’s Len Britton is a possible exception). Hard-right Tea Party challenger Christine O’Donnell knocked out climate activist Mike Castle on Tuesday, leaving a GOP slate of conspiracy theorists and know-nothings angling for the United States’ highest legislative body. Here are the pivotal climate races this November (along with the latest 538.com estimated likelihood of a Republican pickup):

Pennsylvania: Joe Sestak vs. Pat Toomey

538.com estimate: 91 percent likelihood of Republican pickup

The race to replace moderate-Republican-turned-moderate-Democrat Arlen Specter, a semi-reliable vote for climate action, involves Rep. Joe Sestak (D) — a strong supporter of climate action — against frontrunner Pat Toomey, who believes there is “much debate in the scientific community as to the precise sources of global warming.”

Colorado: Michael Bennet vs. Ken Buck

74 percent likelihood of Republican pickup

Sen. Michael Bennet (D), appointed to fill the seat left by Secretary of the Interior Ken Salazar, is attempting to win his first election against Weld County District Attorney Ken Buck. Bennet supports climate legislation, while Buck has said, “While I think the earth is warming, I don’t think that man-made causes are the primary factor.”

Washington: Patty Murray vs. Dino Rossi

54 percent likelihood of Republican pickup

Climate activist Sen. Patty Murray (D), first elected in 1992, is in a tight race with real-estate businessman Dino Rossi, who believes “there’s still a lot of debate going on this, we see it out there and there’s going to be a big debate going on for the next two, three years.”

Nevada: Harry Reid vs. Sharron Angle

46 percent likelihood of Republican pickup

Senate Majority Leader Harry Reid (D) — who believes “global warming is ruining our country” — is being challenged by Tea Party favorite Sharron Angle, who does not “buy into the whole … man-caused global warming, man-caused climate change mantra of the left.”

California: Barbara Boxer vs. Carly Fiorina

42 percent likelihood of Republican pickup

Sen. Barbara Boxer (D), who as head of the Environment and Public Works Committee has championed strong climate legislation, is in a tight race with former Hewlett Packard CEO Carly Fiorina, who is “not sure” that climate change is real and supports Proposition 23, the oil-company effort to overturn California’s climate policy.

Wisconsin: Russ Feingold vs. Ron Johnson

36 percent likelihood of Republican pickup

Sen. Russ Feingold (D) — who believes “we must work to reduce our emissions of greenhouse gases” because “global warming will have serious and possibly irreversible impacts in both the near and distant future” — is running for reelection against Ayn Rand acolyte Ron Johnson, who thinks global warming is “just sunspot activity.”

The rest of the races are unlikely to significantly change the Senate climate policy math. Some races involve long-shot deniers against safe incumbents, others involve the reelection of standing climate opponents, and some mean the replacement of a pro-pollution Democrat with an even more pro-pollution Republican — such as Blanche Lincoln’s and Evan Bayh’s seats.

The greatest determining factor for climate policy in the U.S. Senate is majority control, which determines what legislation moves through committee and onto the floor, and what issues are investigated. An Inhofe-led Environment and Public Works Committee would mean hearings about the “global warming hoax” instead of markups of climate legislation. Even if Democrats retain control of the Senate, which 538’s Nate Silver estimates is a 79 percent likelihood, the committee ratios will be adjusted to reflect lost seats, making it more likely that voting blocs of Republicans and anti-climate Democrats could overwhelm progressives on key votes.