Pope Francis' popularity in the United States has dropped this year. (File Photo)

SOUTH BEND, IND. (CNA/EWTN News) - Most Americans have a favorable view of Pope Francis, but an apparent decline in their numbers may be due to his implicit challenge to American culture, one observer has said.

"Americans never react well when an international leader criticizes their culture," Dr. Kathleen Cummings, a University of Notre Dame professor of American Studies, told CNA July 24. "Given that Pope Francis has done so implicitly in his recent remarks and writings, it is not surprising to see that his approval ratings have declined in the United States."

A Gallup survey of U.S. adults in early July found that 59 percent had a favorable view of Pope Francis. This is a 17 percent decline from February 2014, when 76 percent of Gallup respondents said they had a favorable view of the Pope.

The latest figure resembles poll results in April 2013, a month after the Pope's election, when 58 percent of Gallup respondents said they had a favorable opinion of Francis.

The favorability of Catholics in the U.S. toward the Pope has also declined. In February 2014, about 89 percent of self-identified Catholics told Gallup they had a favorable view of Pope Francis. In July 2015, this figure dropped to 71 percent, a drop of 18 percent. About half of non-Catholic Christians now say they have a favorable view of Francis, compared to 73 percent in 2014.

The percentage of self-identified conservatives who reported favorable views of Pope Francis dropped from 72 percent in 2014 to 45 percent in 2015. The percentage of liberals reporting favorable views dropped from 82 percent to 68 percent, while moderates dropped from 79 to 71 percent.

The Pope's overall unfavorable rating is only about 16 percent, a slight rise from nine percent in 2014, and 10 percent in 2013. About 25 percent of all July 2015 respondents said they had never heard of Pope Francis or had no opinion of him, compared to 16 percent in 2014.

The July Gallup poll came soon after the release of Pope Francis' encyclical on care for our common home, Laudato Si'. The lengthy work spoke of the need to care for God's creation. It also backed several remedies for what the Pope characterized as an environmental crisis.

He said there was a "very solid" consensus on climactic warming.

"Humanity is called to recognize the need for changes of lifestyle, production and consumption, in order to combat this warming or at least the human causes which produce or aggravate it," the Pope wrote.

Cummings, who directs the Cushwa Center for the Study of American Catholicism, said Pope Francis is like previous Popes in "calling upon U.S. Catholics ... to live their faith in ways that run counter to the national culture."

"The difference is that he has highlighted issues that tend to challenge those who identify as conservatives, which explains the steeper decline among their approval ratings," she said.

The Gallup telephone survey of 1,009 U.S. adults took place July 8-12. It claims a margin of error of plus or minus four percentage points.

Benedict XVI's favorability peaked in 2008 at 63 percent, but had dropped to 40 percent in 2010, according to Gallup's surveys. Pope John Paul II's favorability peaked at 86 percent in 1998, and never dropped lower than 61 percent.