The president of a top environmental group that endorsed Hillary Clinton explains why he's still going to challenge her on issues

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The unexpected strength of Sen. Bernie Sanders' candidacy has emboldened some advocacy groups supporting Hillary Clinton to push for a more robust progressive agenda.

On Thursday, the Sierra Club, one of America's oldest and largest environmental activist groups, threw the support of its hundreds of thousands of members behind Hillary Clinton

“I am honored to have earned the endorsement of the Sierra Club, an organization that has fought to protect our environment, our natural wonders, and the health of our children for more than a century," Clinton said in a statement.

But in an interview with Business Insider later in the afternoon, organization President Michael Brune said that while the group is enthused about a Clinton presidency, it still plans to challenge her on various issues where it feels that she's not moving far enough.

"We are going into this eyes wide open. We firmly believe that Hillary Clinton has a very strong environmental platform, and we also accept that we will need to challenge her and push her to go even further," Brune said. "We’re prepared to do that."

Brune noted that the group will push Clinton to embrace a trade agenda that "would actually help the climate movement and accelerate progress, rather than simply not undermine it." The Sierra Club president also praised Clinton's move to the left on hydraulic fracturing, but said she could be "to be even tougher" on the process in the future.

The Sierra Club's endorsement came after months of debate between climate activists over whether to embrace Clinton's more pragmatic proposals to curb the effects of climate change or Sanders' more radical ideas.

While the former secretary of state has laid out a climate change agenda that goes further than President Barack Obama's, for many environmental activists, Sanders' ambitious plan to combat climate change served as a rallying point.

With little regard for the opposition to curbing climate change from Congressional Republicans, Sanders unveiled a plan that would tax carbon emissions, ban offshore drilling, and eliminate fossil fuel subsidies. Environmental activists cheered when Sanders stated onstage at an early Democratic debate that climate change was the biggest threat facing the US today. The Sanders campaign itself criticized Clinton's climate plan as vague.

Groups that endorsed Clinton saw a swift backlash from some members who believed that Sanders' plan was more comprehensive.

Some League of Conservation Voters Action Fund supporters threatened to withhold future donations after the group endorsed Clinton in November. While climate group 350 action did not endorse a candidate, some of its members tracked both the presumptive Democratic presidential nominee and her husband, former President Bill Clinton, peppering them with tough questions about climate change on the campaign trail. 

Friends of the Earth Action endorsed Sanders early in the 2016 race after Clinton failed to say that she would not approve the Keystone XL pipeline, a controversial pipeline that would have funneled oil from Canadian tar sands to the Gulf coast. The group aired pro-Sanders ads in several early primary states. Clinton eventually came out against the pipeline.

The Sierra Club decided earlier this year against endorsing a candidate to avoid taking a side in the rift between environmental activists during the Democratic race.

But with the Democratic primary wrapping up, top climate activists suggest that the ideological gulf between Clinton and presumptive Republican presidential nominee Donald Trump — who has claimed that climate change is a Chinese hoax — will be more than enough to motivate "climate voters" to support the former secretary of state.

"The act of running for president tends to exaggerate differences," Brune told reporters in New York in April.

"The choice between Donald Trump and Hillary Clinton for environmentalists is the easiest it has ever been in history," Brune said on Thursday. "We have in Hillary Clinton a candidate who has the strongest set of policies of any presidential nominee in history and would be the first woman elected as president in history, contrasted with a candidate who calls climate change a hoax a con job a concept created by the Chinese, and who has delighted in dividing Americans against each other based on race and gender and physical ability."

As a record number of Americans see climate change as a serious threat, groups like the Sierra Club are planning to take a more active role in the 2016 election.

While in past elections it has invested in television advertisements, this cycle, the group is investing its resources in mobilizing volunteers, and directing much of its political activism to knocking on doors and other grassroots get-out-the-vote efforts.

"People trust their friends and their neighbors and we’ll have Sierra Club volunteers knocking on doors, making phone calls in districts all across the country," Brune said. "We’ll have tens of thousands of volunteers who are engaging this election, because we think that back to the roots between friends and neighbors is the way in which you win."

By pointing to the growing clean energy business sector, Brune hopes to buck the popular belief in political circles that voters will not be motivated to head to the ballot box to weigh in on climate change issues. The Sierra Club president said even in the last four years, jobs like wind turbine technician and solar panel installer have become some of the fasted growing jobs in the economy, while companies like Tesla have made clean energy products into status symbols.

"What has happened in the last few years is that we have found people linking climate change to other things that they care about and care about strongly," Brune said in April.

He added: "The politics have never been better."

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