Pope Francis' popularity among conservative Americans has dropped considerably since last year primarily due to his views regarding immigration and climate change, a new Gallup pole has found.

The pontiff's favorability rating in the United States dropped from 76 percent in early 2014 to 59 percent this month, Gallup reported this week. This 17 percentage-point decline puts his approval rating close to the 58-percent rating he received in April 2013, shortly after his papal inauguration.

Catholics and political conservatives, two groups that have been "ardent supporters of the modern papacy," drove the rating down. Among these groups, 71 percent of Catholics think of Pope Francis favorably, compared to 89 percent last year. Moreover, 72 percent of conservatives approved of the pope last year, whereas only 45 percent approve of him now.

Gallup contends that the dramatic shift among conservatives "may be attributable to the Pope's denouncing of 'the idolatry of money' and linking climate change partially to human activity, along with his passionate focus on income inequality-all issues that are at odds with many conservatives' beliefs."

According to Huffington Post reporter Paul Vale, the Pope's decline in popularity is also due in part to his approval of the United States' nuclear deal with Iran- "an agreement most Republicans decried"- and his views on immigration.

During a border crisis over the summer in which more than 60,000 unaccompanied minors crossed illegally into the United States, nearly all from Central America, Pope Francis urged the U.S. to welcome the child migrants.

"This humanitarian emergency requires, as a first urgent measure, these children be welcomed and protected. These measures, however, will not be sufficient, unless they are accompanied by policies that inform people about the dangers of such a journey and, above all, that promote development in their countries of origin,"  Francis said last year, drawing criticism from Republican presidential candidates Jeb Bush and Rick Santorum.

Princeton Professor Robert George, a prominent Catholic conservative thinker, told CBS News that the decline Francis' popularity should not be surprising, because he is an outspoken leader. "Given that he leads a Church that stands for certain things -- things that are controversial and rejected by some -- his popularity was bound to fall as he moved forward in his pontificate proclaiming the truths of the faith." 

Despite the slump in numbers, Pope Francis' 59 percent favourable rating is still higher than the 40 percent favorability rating of Pope Benedict XVI in 2010 before he retired in 2013.

"Pope Francis is still viewed favourably among Americans, but his image has declined since early 2014. The decline in his favourable rating reflects, in part, the increase in the percentage of Americans who don't have an opinion of the pope, but also a sharp drop in favourable opinions among Catholics and political conservatives," Gallup notes.

Pope Francis will make his first visit to the U.S. in September, stopping in Washington, D.C., New York and Philadelphia, and will be the first Pope to address a joint session of Congress.