(CNN) Hillary Clinton has embraced Barack Obama's legacy. Her campaign chairman is a former White House adviser credited for pushing through pro-environmental regulations. And she talks about the need to act on global warming.

But in this Democratic primary, that's not enough. Her critics -- and even would-be supporters -- continue to push a message the Clinton really can't be trusted to do the green thing.

"On Day 1, I said the Keystone pipeline is a dumb idea," Bernie Sanders said at CNN's Democratic Presidential Town Hall on Monday night. "Why did it take Hillary Clinton such a long time before she came into opposition to the Keystone pipeline?"

The former secretary of state being hit on environmental issues is another example of her playing defense on the left against Sanders. The message: She's late to the liberal position on the environment, trade, Wall Street, Iraq, etc., so can she truly be trusted?

Liberal megadonor Tom Steyer, who spent $70 million backing Democratic candidates in 2014, isn't satisfied with Clinton just yet. Last week, he sent a clear and public warning to Clinton , saying he is looking at a Sanders endorsement because he wants to hear more from the former New York senator.

"I don't think she's fully fleshed out everything she has to say about energy and climate," Steyer told Reuters. "I think that as the campaign goes on I would imagine she will put out more detailed plans of exactly what she thinks. I don't find what she's said inadequate, but I don't think it's complete yet."

That message fits in with what Sanders has been telling voters: He's there first, while you have to wait for Clinton.

Sanders and his environmental allies are airing ads touting his early anti-Keystone stance, and he campaign last week with the leading anti-Keystone crusader, Bill McKibben

By comparison, as secretary of state in 2010 , Clinton said she was "inclined" to approve Keystone. Clinton eventually said last year she opposed the pipeline, and President Barack Obama killed the proposal in November . Yet in the meantime, the project became such a cause célèbre for Democrats that it still resonates on the campaign trail.

At the Democratic town hall, Sanders also cited the massive Trans-Pacific Partnership trade deal, which Clinton eventually opposed as well.

"I have understood from Day 1 that our trade policies have cost us -- NAFTA, CAFTA, PNTR with China, millions of decent-paying jobs," Sanders said. "I didn't have to think hard about opposing the Trans-Pacific Partnership."

"It took Hillary Clinton a long time to come on board to that," he added.

To Clinton's campaign, this is hogwash.

When Sanders spokesman Michael Briggs piggybacked on Steyer's interview and questioned where her climate plan was, a top Clinton adviser responded with a Medium post

"The Sanders campaign asked where our climate plan was. I Googled it for them," wrote John Podesta, Clinton's campaign chairman and a former top adviser to Obama who pushed a strong environmental agenda, last week.

Clinton's proposal includes incentives to have "more than half a billion solar panels installed across the country by the end of her first term," and to "generate enough clean renewable energy to power every home in America within 10 years of taking office," Podesta wrote.

He also cited Clinton's backing of Obama's Environmental Protection Agency's greenhouse gas regulations and opposition to oil drilling in the Arctic and the Atlantic Ocean.

But Clinton isn't proposing a price on greenhouse gas emissions, or so-called carbon tax, unlike the other two Democrats.

Such a plan is likely a non-starter on Capitol Hill regardless. A cap-and-trade bill for carbon dioxide emissions passed the House in 2009 but died in the Senate the next year, and the conservative backlash against the measure is credited with helping to spur the tea party movement.

'The planet would literally burn up'

Clinton has not gone as far as Sanders, who endorses cutting greenhouse gas emissions 80% below 1990 levels by 2050 and putting a tax on carbon emissions. And Steyer last year praised long-shot Democratic candidate Martin O'Malley's plan as well -- another un-subtle dig at Clinton.

O'Malley, who drew loud applause at the town hall when he brought up the issue of climate change, simply tells interviewers that the planet is doomed should Clinton wins.

"I don't believe that her plan is very serious," O'Malley said in an interview with Climatewire on Clinton's agenda . "I believe that it's an incrementalist approach. And an incrementalist approach is not going to get us to where we need to go, nor is it going to claim this tremendous business opportunity for the United States."

The former Maryland governor told The New Republic that Clinton's plan amounted to "a voluntary solar panel plan for residences."

"Under her plan the planet would literally burn up," O'Malley said.