The issue of obedience has weighed on those nuns of late, as the Vatican has deemed the women on stage with the vice president radical feminists who pay too much attention to social justice and too little to promoting church teaching on issues like abortion and same-sex marriage.

Mr. Biden’s visit came just weeks after Cardinal Gerhard Müller, the Vatican’s enforcer of doctrine, shocked many American nuns with his comments to a Vatican newspaper. As he defended the effort to rein in the nuns, the cardinal remarked, “Above all, we have to clarify that we are not misogynists; we don’t want to gobble up a woman a day!”

So while political reporters focused on Mr. Biden’s appearance as his first foray back into a presidential campaign season, the outing also put the nation’s first Roman Catholic vice president in the middle of a protracted political fight between the pope he admires and the American nuns he reveres.

“All politics begin here in Iowa,” said Sister Simone Campbell, the head of Network, the group that organized the tour and described Mr. Biden’s papal conversations. She expressed delight that the vice president had lent some star power to what she called “our little, teeny event.”

In an interview, Sister Campbell said Mr. Biden had expressed a willingness to join the nuns after their first tour in 2012, “Nuns on the Bus: Nuns Drive for Faith, Family and Fairness.” That was the year that the Vatican’s Congregation for the Doctrine of the Faith cracked down on the Leadership Conference of Women Religious, an umbrella group that represents about 80 percent of America’s 57,000 nuns. The report explicitly cited Sister Campbell’s group, which helped lobby for President Obama’s health care law, as being a particularly bad influence.