WASHINGTON — Since his first homily in 2013, Pope Francis has preached about the need to protect the Earth and all of creation as part of a broad message on the environment. It has caused little controversy so far.

But now, as Francis prepares to deliver what is likely to be a highly influential encyclical this summer on environmental degradation and the effects of human-caused climate change on the poor, he is alarming some conservatives in the United States who are loath to see the Catholic Church reposition itself as a mighty voice in a cause they do not believe in.

As part of the effort for the encyclical, top Vatican officials will hold a summit meeting Tuesday to build momentum for a campaign by Francis to urge world leaders to enact a sweeping United Nations climate change accord in Paris in December. The accord would, for the first time, commit every nation to enact tough new laws to cut the emissions that cause global warming.

The Vatican summit will focus on the links between poverty, economic development, and climate change, with speeches and panel discussions by climate scientists and religious leaders, and economists like Jeffrey Sachs of Columbia University. UN Secretary-General Ban Ki-moon, who is leading efforts to forge the Paris accord, will deliver the opening address.

In the United States, the encyclical will be accompanied by a 12-week campaign, now being prepared by a committee of Catholic bishops, to raise the issue of climate change and environmental stewardship in sermons, homilies, news media interviews, and letters to newspaper editors, said Dan Misleh, executive director of the Catholic Climate Covenant in Washington.

But the effort is already angering a number of American conservatives, among them members of the Heartland Institute, a libertarian group partly funded by the Charles G. Koch Foundation, run by the billionaire industrialist Koch brothers, who oppose climate policy.

“The Holy Father is being misled by ‘experts’ at the United Nations who have proven unworthy of his trust,” Joseph Bast, the president of the Heartland Institute, said in an interview. “Though Pope Francis’ heart is surely in the right place, he would do his flock and the world a disservice by putting his moral authority behind the United Nations’ unscientific agenda on the climate.”

The group is planning protests against Francis on Tuesday in Rome.