WASHINGTON (AP) - Top Democratic presidential contenders talked tough on Wednesday (Sept 4) on cutting climate-damaging emissions from oil, gas and coal, turning their focus to global warming in a marathon evening of town halls that gave the candidates a chance to distinguish themselves on a topic of growing importance to their party's liberal base.

The lengthy climate conversations promised to hand Republicans ammunition for next year's general election fight by emphasising one common element in the Democrats' climate change plans: their overwhelming - and overwhelmingly costly - scope.

But the 10 Democrats who participated in the seven-hour series of climate change forums on CNN did not shy away from making sweeping promises to reshape the American economy in service of what their party's grassroots supporters see as the paramount goal of averting global warming's most devastating effects.

"We have a moral responsibility to act and act boldly. And to do that, yes, it is going to be expensive," said Vermont Senator Bernie Sanders, who billed his US$16 trillion (S$22 trillion) climate change plan as a necessary response to scientists' calls for dramatic cuts to carbon emissions.

Former vice-president Joe Biden took a more pragmatic view than Mr Sanders, even as he defended his own climate proposal as "aggressive enough" to meet the challenge.

Mr Biden, who has held an early lead in the Democratic primary, has pledged to regulate the oilfield production method known as hydraulic fracturing - though not abolish it, as some rivals have - and said on Wednesday that he doubted an outright ban could be feasible.

After facing sharp questions about his plans to attend a Thursday fund raiser hosted by the co-founder of a natural gas company, Mr Biden defended his decision as consistent with a pledge he signed to turn away any contributions from fossil fuel executives or lobbyists.

The energy investor in question, his former aide Andrew Goldman, is described in a company press release as "a long-term investor in the liquefied natural gas sector".

California Senator Kamala Harris vowed that, as president, she would ban hydraulic fracturing, also called fracking, and take other steps to cut fossil fuel emissions, regardless of whether Republicans cooperated.

Ms Harris said she would eliminate the Senate filibuster, if necessary, to get liberal Democrats' sweeping Green New Deal proposal passed with a simple majority vote - a significant move from a candidate who had previously stopped short of a pledge to change congressional procedure.

In targeting oil and gas and coal production, "this is a fight against powerful interests", Ms Harris said. "It's lead, follow or get out of the way... starting with Mr Donald Trump."

Mr Sanders declined to support a full end to the filibuster, asserting that he could get climate change legislation through Congress without needing to eliminate the Senate's 60-vote requirement for many Bills by using a procedural manoeuvre that the Republican Party most recently used in 2017 to pass massive tax legislation.

All 10 candidates have proposed plans starting at US$1 trillion for investment and research designed to wean the US economy off oil, gas and coal by mid-century, with varying focuses on sharp emissions cuts and technological solutions, among other measures.

Former housing secretary Julian Castro led off the town halls, defending his own decision to stop short of endorsing a national fracking ban by saying that natural gas - some of it from fracking - had served as a bridge while the economy moves to renewable energy sources like solar and wind.

He cited the extreme weather over the summer to illustrate the urgency of the moment.

"We see that now with Hurricane Dorian," said Mr Castro, who joined Mr Biden and Minnesota Senator Amy Klobuchar in calling for regulating but not completely ending fracking.

"The Arctic ice caps melting. The Amazon on fire."

US President Donald Trump began tweeting almost as soon as the forums kicked off, calling Democrats' proposals to address climate change unnecessary and costly.

"The Democrats' destructive "environmental" proposals will raise your energy bill and prices at the pump," Mr Trump warned.

Candidates suggested, at turns throughout the night, an array of smaller-scale ideas to limit emissions - from banning plastic straws to adjusting dietary guidelines to washing clothes in cold water.

Massachusetts Senator Elizabeth Warren, for her part, brushed aside questions about those lifestyle changes by noting that the resulting public debate is "what the fossil fuel industry wants us" to engage in while they remain considerable contributors to carbon emissions.

Mr Pete Buttigieg spoke broadly about addressing climate change not just as an economic issue but also as a moral and national security imperative.

The Afghanistan War veteran proposed that the military should be "leading the way" in solving the issue, by making sure bases are carbon neutral and by purchasing zero-emissions vehicles, among other things.

Democrats spent the run-up to the town halls burnishing their environmental credentials, with five candidates releasing in-depth proposals to slash carbon emissions.

Mr Sanders went further, challenging his rivals for the party's presidential nomination to join him in supporting a full ban on fracking, which is strongly opposed by most environmentalists who view it as an unmanageable risk to local water and air quality as well as the broader climate.