Pope Francis' favourable rating among Americans has dropped considerably since last year ahead of his visit to the United States in September, according to a Gallup poll.

According to the survey, conducted from July 8-12, 59 percent of Americans have favourable opinion of the Pope, down by 17 percentage points from 76 percent in February last year.

A total of 16 percent rated him unfavourable, up from 9 percent last year. Those who said they never heard of him or didn't have any opinion totalled 25 percent, from 16 percent last year.

Gallup said the drop in the Pope's favourable rating is a result of a "decline among Catholics and political conservatives, two groups that have been ardent supporters of the modern papacy."

A total of 71 percent of Catholics had favourable rating of the Pope, down from 89 percent last year.

Protestants and other Christians rated him favourably at 52 percent, a decrease from 73 percent last year.

Pope Francis got favourable rating of 45 percent from conservatives, 71 percent from moderates and 68 percent from liberals, down from 72 percent, 79 percent and 82 percent, respectively.

Gallup said the decline 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."

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

On the other hand, Pope John Paul II, who served for 27 years, had a favourable rating of always above 60 percent, even reaching 86 percent in 1998 among Americans.

"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," according to Gallup.

The Pope will visit New York, Philadelphia and Washington, D.C. in September and will be the first Pope to address a joint session of Congress.

Gallup polled 1,009 adults aged 18 and above in all 50 states and the District of Columbia for the survey.