Hillary Rodham Clinton, former secretary of state, speaks during her keynote remarks at the Global Alliance for Clean Cookstoves summit on Nov. 21 in New York. (Bebeto Matthews/AP)

Hillary Rodham Clinton’s environmental problem was on full display in back-to-back events here Monday evening.

First, Clinton appeared at a private fundraiser for Sen. Mary Landrieu (D-La.), an embattled incumbent who favors construction of the massive Keystone XL oil pipeline opposed by environmentalists.

Just two hours later, Clinton appeared on a different stage to deliver a speech to the League of Conservation Voters — a mainstream environmental group that strenuously opposes Keystone.

“The science of climate change is unforgiving,” no matter the continued skepticism about its causes, Clinton said. “The political challenges are also unforgiving. There is no getting around the fact that the kind of ambitious response required to combat climate change is going to be a tough sell at home as well as around the world.”

She did not mention the pipeline that Landrieu insists is essential to Louisiana.

Clinton, left, waves to the crowd as she appears on stage to speak at a campaign event for Sen. Mary Landrieu (D-La.) in New Orleans on Nov. 1. (Gerald Herbert/AP)

The contrasting events illustrate the dilemma facing Clinton, who has declined to take a position on Keystone but needs support from the Democratic Party’s crucial environmental wing if she pursues a presidential bid in 2016.

Environmentally minded donors and voters are likely to hold greater sway among Democrats in the 2016 presidential election than they did when Clinton ran and lost in 2008. Her refusal to take a stand on Keystone has disappointed some of the loudest — and richest — environmental activists who view the project as a test of a candidate’s environmental bona fides.

The brief speech to the League of Conservation Voters — at a gala dinner — was Clinton’s first policy-related public appearance since the midterm elections and came amid growing evidence that she will soon announce whether she will pursue a White House run.

In isolation, her appearance at the league event could be construed as at least tacit opposition to the Keystone project. But her appearance with Landrieu muddies the question: The Louisiana Democrat, who faces stiff odds in a runoff election against Rep. Bill Cassidy (R-La.) on Saturday, says Keystone will help create energy jobs in her state, and she spearheaded a failed effort to pass Senate legislation approving it.

Clinton, who also stumped for Landrieu during the regular midterm campaign, addressed donors paying as much as $12,000 a ticket for cocktails at the home of a longtime Clinton supporter. The event was closed to the news media.

“We are thrilled that Secretary Clinton is speaking at our event,” league spokesman Jeff Gohringer said when asked about the juxtaposition with the Landrieu party. The group “has no control over her schedule,” he said.

A spokesman for Clinton said she had no comment on Keystone.

Although similar projects have gone ahead with little objection, opposing the Canada-to-U.S. pipeline has become a mark of environmental commitment among activists. The State Department has jurisdiction because the pipeline crosses an international border, and Clinton has said that her four years as secretary of state make it inappropriate for her to weigh in.

President Obama says a State Department review should run its course before a decision is made, meaning that Clinton could be forced to choose a side if Keystone remains unresolved far into 2015. She has said she is likely to make up her mind about running after Jan. 1.

Clinton was not paid for the league speech but has added several speeches to her January and February schedule, indicating that she will continue her lucrative paid speaking career into the new year. Clinton is set to deliver two speeches in Canada at events sponsored by the Canadian Imperial Bank of Commerce, and to address the Watermark Silicon Valley Conference for Women on Feb. 24.

Julia Ballantyne Wright, a spokeswoman for the Watermark event, would not say whether Clinton’s appearance was paid. “The Conference team does not comment on arrangements between the Conference and its speakers,” Wright said in an e-mail.

Clinton’s tour on the paid speaking circuit, which pays her $200,000 or more per appearance, has drawn controversy. At least eight universities, including UCLA and three other public institutions, paid Clinton to visit their campuses and speak to students, faculty members and other guests.

Keystone reflects the increased clout of voters and donors animated by environmental issues. California billionaire and prolific Democratic donor Thomas Steyer, who had pledged to spend $100 million during the 2014 midterms, has been blunt in saying that he has made the pipeline an uncomfortable subject for his political allies. Steyer’s spending came in under $100 million, however, and produced few tangible results for Democrats.

Nonetheless, appealing to what one senior Democrat calls the “Steyer wing” may be particularly important for Clinton now, assuming she wants to quietly outmaneuver more liberal potential challengers for the Democratic nomination.

Steyer has not publicly endorsed her, although his older brother is a close Clinton ally. Former Bill Clinton White House aide Chris Lehane, a Steyer political adviser, said Steyer is committed to a “multi-cycle process” and expects to see the same environmental issues resonate in 2016.

Steyer aside, the issues surrounding climate change and energy production and usage have become sharper in many Democratic voters’ minds since 2008.

The oil-drilling practice of hydraulic fracturing, or fracking — whether viewed as a potential economic boon or a potential environmental hazard — was barely in the public eye when Hillary Clinton last ran.

“Climate change and energy played a big role in 2012, and we have seen an increase through 2014 and heading into 2016,” the league’s Gohringer said.

His group had record spending of more than $30 million on election-related activities in the midterm cycle, Gohringer said. He declined to give an estimate for what Clinton would raise for the organization Monday.

The league endorsed Clinton for New York Senate in 2000 and 2006. The group endorsed Barack Obama for president in 2008.

Philip Rucker contributed to this report.