Democratic voters are using climate as a litmus test for 2020 candidates — who are proposing solutions that would have been viewed as politically unthinkable just four years ago.

The candidates are outdoing one another with multitrillion-dollar plans in a bid to win over progressive voters. These expansive proposals, which go far beyond the policies of President Barack Obama, could make Democrats less palatable to the blue-collar and Midwestern voters who fear the impact of tough climate rules on jobs and the economy.

"The policies that are being advocated on the left right now may be alienating the voters they are attempting to reach," said Paul Bledsoe, who advised President Bill Clinton on climate issues and now works as an adviser with the Progressive Policy Institute. "Candidates on the left are outbidding each other on the costs and ambitions of their plans to appeal to climate activists."

Mike McKenna, a GOP energy strategist and pollster, said the plans were a "tremendous gift" to Republicans.

"People who want to use this issue to grow government or signal virtue or whatever have not made the case," McKenna said. "No one has convinced voters that their fear of climate change is greater than their fear of government action."

The latest to enter the fray is former Vice President Joe Biden. He released his plan Tuesday after criticism from his Democratic rivals, who tried to paint him as weak on the issue and beholden to Obama administration policies they consider too timid. Biden, who is campaigning on rebuilding the middle class and whose policies generally hew more to the political center, came under attack last month after one of his aides reportedly told reporter that he would find a "middle ground."

The Biden campaign dismissed the report as inaccurate. That hasn't stopped some of his rivals from using it against him. Speaking to the California Democratic Party's convention over the weekend, Sen. Bernie Sanders of Vermont reprised the "middle ground" phrase to implicitly criticize the former vice president without naming him. "We have got to make it clear that when the future of the planet is at stake, there is no 'middle ground.' "

Biden's long-awaited proposal may defuse some of the criticism from the left. Calling climate change "an existential threat," Biden proposed achieving 100% clean energy and net-zero carbon emissions by 2050 as part of a $5 trillion plan that would be paid for by undoing President Donald Trump's tax overhaul.

Biden also vowed to refuse any donations from fossil fuel companies and executives, joining a pledge from 16 other Democratic candidates.

Sunrise, an environmental group that had criticized what it saw as Biden's weak stance on climate change, applauded the new plan as a "major victory."

For Democrats, elevating climate change into a key issue has advantages. For one, it can allow them to turn it into a liability for Trump, who has dismissed global warming as a hoax. A focus on the issue presents an opportunity to galvanize voters by highlighting the president's inaction and his efforts to roll back environmental regulations.

A Monmouth University poll in May showed that Democratic voters rate climate change as the second-most important issue, after health care. In a national poll conducted last month by CNN, 82% of registered voters who identified as Democrats or Democratic-leaning independents listed taking action on climate change as a "very important" priority they'd like to see get the focus of a Democratic presidential candidate.

The focus on climate stands in stark contrast to the 2016 election, when it wasn't even a topic during any of the three presidential debates.

"It is finally hitting home this isn't happening 100 years from now, it's happening now," said Sen. Amy Klobuchar, D-Minn., a 2020 candidate who was mocked by Trump for talking about climate change during a snowstorm.

Washington Gov. Jay Inslee, who has centered his presidential run around combating climate change, has proposed a $9 trillion plan that he promised would create 8 million jobs and decarbonize the economy. A separate policy proposal from the candidate vowed to achieve net-zero carbon emissions by 2045.

Massachusetts Sen. Elizabeth Warren unveiled a $2 trillion plan on Tuesday that, like Biden's, would aim for zero emissions by 2050 but would be financed by a tax on corporate profits of more than $100 million.