By Richard Valdmanis and Valerie Volcovici

CLEVELAND/WASHINGTON (Reuters) - Republican presidential candidate Donald Trump has called climate change a hoax and has vowed not to let regulation stand in the way of expanded U.S. oil drilling if elected. But a group of his supporters at the Republican National Convention in Cleveland this week are lobbying him to soften that stance.

Youth, religious and investor activists aligned with the Republican Party are working the sidelines of the July 18-22 conclave, arguing that a greener platform could help the New York businessman-turned-politician win voters in November.

"In an age where the Republicans are particularly interested in winning the presidency, they are not supporting an agenda that reflects that most Americans recognize climate change is a problem," said Rachel Lamb, national organizer for the Young Evangelicals for Climate Action group.

Lamb and representatives from other organizations like the Greater Cleveland Young Republicans, environmental advocacy group RepublicEn, and clean energy foundation ClearPath taken part in panel discussions and held private meetings with operatives on the sidelines of the convention.

Trump was formally nominated on Tuesday to compete in the Nov. 8 presidential election against presumptive Democratic nominee Hillary Clinton, who has made combating climate change a key theme in her campaign.

The Republican Party has adopted a platform that calls coal a “clean” source of electricity and would open more federal land to drilling and mining, while Trump has said he would reverse some of President Barack Obama's key climate change initiatives.

Some 64 percent of U.S. adults say they are worried a "great deal" or a "fair amount" about global warming, the highest reading since 2008, according to the results of a Gallup poll released in March.

Cade Marsh, a representative of the College Republican National Committee, said he was hoping to convince Republicans in Cleveland this week that environmentalism and conservatism are not mutually exclusive.

"'Climate change' is a word that is seen as far left," Marsh said. "But if you repackage clean energy in terms of national security and liberty, you'll find people much more receptive," he said, referring to the role of renewable energy sources like wind power and solar power in reducing dependence on foreign oil.

He said his message has been warmly received by convention attendees, but that he held out little hope of a turnaround in Trump's policies. "This is about tomorrow's Republican party," Marsh said.

Jay Faison, a North Carolina Republican donor who founded the ClearPath foundation, said he was telling Republican operatives in Cleveland this week that he would prefer to fund candidates who can put clean energy on the party’s agenda.

"I wonder about the opportunities with the persuadable voters in the middle in terms of the general election,” he said.

(Editing by Jonathan Oatis)