The League of Conservation Voters will spend $25 million in campaign funding this election season, a fivefold increase over what the group devoted to the last midterm elections, LCV President Gene Karpinski said in an interview.

The spending will be largely devoted to key Senate races but also will go to a handful of gubernatorial and state legislative contests. The increased funding reflects the growing role of environmentalists as political money players. Climate activist and billionaire Tom Steyer has already spent $22 million on federal and state candidates this election cycle and plans to devote at least $26 million more. Steyer is a major LCV funder.

“We are poised to make, by far, the biggest investment we’ve ever made in elections,” Karpinski said in an interview, adding that the group’s efforts are “making climate change part of the conversation” in races across the country.

The group has ramped up its spending in recent years, rising from $5 million in 2010 to $15 million in 2012. It also has joined with another major environmental group, the Natural Resources Defense Council Action Fund PAC, to run the GiveGreen program, which has raised or contributed $4 million so far this election cycle to individual federal candidates.

The Environmental Defense Action Fund, which has traditionally only given money directly to candidates, has already spent more than $1 million in federal and state races in Colorado, Iowa, Kansas, Michigan and New York this year.

FTI Consulting senior director Matt Dempsey, whose clients include several fossil-fuel industry interests, questioned whether green groups would be able to sway voters.

“Anti-fossil fuel groups, no matter how much money they spend, face an uphill battle at the ballot box because they simply cannot explain to the public how they plan to meet energy needs without fossil fuels, both now and in the future,” Dempsey wrote in an e-mail.

Dempsey noted that several of the Senate Democrats up for reelection, including Mark Begich (Alaska), Kay Hagan (N.C.) and Mary Landrieu (La.), support the Keystone XL pipeline, which most national environmental groups oppose. LCV is backing Begich and Hagan, as well as Mark Udall (Colo), who describes himself as “a champion of Colorado’s natural gas industry”; the three incumbents support mandatory federal limits on greenhouse gas emissions linked to climate change.

Environmentalists’ deeper involvement in both state and federal campaigns represents, to a large extent, a recognition that legislation curbing greenhouse gas emissions on a broad scale will remain out of reach for years without a major political shift in Washington and state capitals.

Elizabeth Thompson, Environmental Defense Action Fund’s president, said in a statement that her organization is “making a major investment to build a bipartisan movement for environmental progress. . . . It won’t be easy or quick, but we’re convinced that solving the biggest challenges will require both parties at the table. Our goal is to show both sides that good climate policy is smart politics.”

The races LCV is targeting — including Senate contests in Alaska, Colorado, Iowa, Michigan, New Hampshire and North Carolina, as well as the Maine gubernatorial race, where it is opposing Gov. Paul LePage’s (R) reelection, and state legislative races in Oregon and Washington — all involve significant contrasts between the two candidates on climate change and other signature environmental issues.

It has endorsed just four Republicans this cycle — Sen. Susan Collins (Maine) and three state legislators, all of whom faced primaries. It also intervened in two Democratic primaries, successfully backing Sen. Brian Schatz (Hawaii) and Maine state Sen. Emily Cain, who is trying to succeed Rep. Michael H. Michaud (D).

The issue of climate change has come up in several of these races, such as when former senator Scott Brown (R-Mass.), who is challenging Sen. Jeanne Shaheen (D-N.H.), responded to a question of whether “the theory of man-made climate change has been scientifically proven” during a GOP primary debate by saying, “Uh, no.”

Brown spokeswoman Elizabeth Guyton said in a statement that he “believes that the climate is changing by a combination of natural and man-made causes.”

Steyer’s NextGen Climate Action Committee — which is giving money not only to environmental organizations but also to labor, abortion rights, veterans and Latino groups — “will be a seven-figure supporter of our work in 2014,” Karpinski said. The committee has donated $650,000 to LCV’s super PAC this election cycle, which was spent on various races including Massachusetts Sen. Ed Markey’s special election.

“There’s not a day that goes by that someone on our team doesn’t talk to someone on the Steyer team,” Karpinski said.

NextGen Climate Action spokesman Bobby Whithorne wrote in an e-mail that his group is canvassing with LCV “in several states and supporting their efforts on the ground in numerous races. We look forward to working together over the next eight weeks to bring climate change to the ballot box.”

The spike in spending by environmental activists has sparked a response from groups aligned with industry and the GOP. The conservative group American Commitment has run ads in Colorado and Iowa questioning Steyer’s support for Democratic Senate candidates, and groups such as American Crossroads, Americans For Prosperity and the U.S. Chamber of Commerce have run ads on the Keystone pipeline and energy in that state. Groups affiliated with the libertarian billionaire brothers Charles and David Koch have provided financial support for the opponents of all of the Senate candidates LCV is backing, a fact it has highlighted in five separate ads in four states.

Some of the ads LCV has run so far, such as those attacking Iowa GOP Senate candidate Joni Ernst, address policies on education as much as the environment. Dan Weiss, LCV’s senior vice president for campaigns, said the group highlighted Ernst’s support for eliminating the Education Department and Environmental Protection Agency because “we want to make it clear to Iowans that she doesn’t share their priorities.”

While the ads have been the most visible sign of green groups’ spending, LCV will devote many of its resources to grass-roots efforts. Weiss said the group will have 2,000 people working in 19 offices and will contact 750,000 voters who typically don’t vote in off-year elections in Alaska, Colorado, Iowa, New Hampshire and North Carolina.