Pope Francis’s arrival in Washington on Tuesday has reinforced hopes that one of the last great bastions of climate change denial – the US Congress – may be on the verge of crumbling.

As the pope touched down in the US from Cuba, Democratic leaders in Congress and environmental campaigners were optimistic that Francis would keep the focus on his core themes of the global economic order, poverty and environmental degradation over the next six days, and so widen the emerging fractures in the Republican wall of denial.

Republicans, meanwhile, remained wary, and expressed hope that the pope would steer clear of controversial issues on his first visit to the US.

Sheldon Whitehouse, the Democratic senator from Rhode Island, said he believed the call to action from a popular pope made it increasingly difficult for Republicans to continue to dismiss the science on climate change. “I think this whole edifice of climate denial is crumbling,” Whitehouse told the Guardian.

As Pope Francis was taking his first steps on American soil, Hillary Clinton broke her lengthy silence on the construction of the Keystone XL pipeline, saying she opposed the project and did not believe it was in the interests of “what we need to do to combat climate change”.

Even before the pope’s indictment of capitalism and climate change in his encyclical in June, Republicans in Congress had been slowly edging away from outright climate denial.

Fifteen Republicans in the Senate voted last January that human activity contributed to climate change. Last week, 11 Republicans in the House broke with the party leadership to acknowledge the threat of climate change.

Whitehouse argued that strong concern about climate change among Latino and young voters made it increasingly difficult for Republicans to cling to climate change denial – and remain viable in the 2016 elections.

“You are seeing folks fleeing it on the House side. There are innumerable behind-the-scene conversations happening on the Senate side with Republican senators who are in real disagreement with where their party has been led and I think it’s only a matter of time until there is a break in the Senate,” he said.

“The forces that are coming to bear on that, of [the] Paris [United Nations Climate Change Conference], of corporate pressure, of key constituencies for the November 2016 elections are all very strong, and the risk to an out-of-touch party going into a national election with an unpopular position on a major issue that has been paid for by a big special interest is an ugly combination. So it is very logical, I think, for people to want escape from that ship before it sinks too much further and to want to escape soon. I think the pressure of the pope talking about this helps.”

That was the hope of campaigners who have been organising around the pope and his visit to the US for months, seizing on the encyclical as a chance to reframe climate change as a question of morality.

Sally Bingham, a reverend who organised 18,000 churches, synagogues and mosques in the Interfaith Power and Light campaign group, said she hoped the pope would help Republicans reposition on climate change denial.

“Maybe the pope will be able to make them think about it a little bit and shift away from the next campaign, and the money that they may be needing, and to think about what kind of leader they are going to be,” she said.

Within Congress there was hope from Democrats that Francis’s visit would provide cover to moderate Republicans who might be willing to back legislation on energy and climate change. “The pope will be helpful in bringing some of the moderates to us,” Ruben Gallegos, a Democratic house member from Arizona, told a conference call.

But he said he doubted the pope’s message would succeed in heading off Republican efforts to defeat the clean power plant rules. “I don’t believe it will change Congress right away,” he said. “Unfortunately the attack on the president’s efforts to mitigate carbon emissions from coal power plants will continue from that wing of the Republican party.”

“Whoever the presidential candidate is, is extremely likely to have an epiphany once he is through the convention and actually is the nominee,” Senator Whitehouse said.

For the moment, however, Republicans in Congress remain sceptical about the visit becoming an opportunity to bash them on a host of contentious issues from climate change to Cuba and economic inequality.

On the eve of the pope’s visit, Paul Gosar, a Catholic Republican from Arizona, said he would boycott Francis for promoting what he called “questionable science”.

Iowa senator Joni Ernst said it was “premature” for Democrats to claim the pope as one of their own. “I am hoping the visit is not politically motivated,” she told the Guardian. “What I want to hear is just some uplifting message about how we can all help the most vulnerable in our society.”

“I think his message is going to be somewhat less political than people believe it will be,” agreed Senator Susan Collins of Maine. “I don’t see him as a person who is trying to influence votes, he is outlining what he believes the mission of the church is in the modern world.”

Some Democrats were quick to seize on the pope’s past criticism of unfettered free markets as vindication for their own focus on income inequality and union pay campaigns.

“As we welcome Pope Francis to the United States, I hope that every member of Congress and the president will heed his call for social and economic justice,” presidential candidate Bernie Sanders told striking federal workers near the US capitol.

“The United States of America today is the richest country in the history of the world, but most Americans do not understand that or feel that because most of the wealth is going to a handful of people at the top,” he added at a packed rally of union members held to coincide with the pope’s arrival.

But on a day when the Senate narrowly voted against moving forward on anti-abortion legislation, others were braced for difficult moral questions for both Republicans and Democrats when the pope addresses a joint session of Congress on Thursday.

“There is a way to be political without being partisan,” said John Gehring, Catholic Program Director at Faith in Public Life, an advocacy group in Washington, during a preview event at the Center for American Progress.

“In the meat-grinder of American politics we chop up issues of life and dignity and social justice, and the pope reflects a Catholic social tradition that connects those dots and he will make people uncomfortable on both sides.”