The media-savvy ‘Nuns on the Bus’ tour kicked off in the midwestern state of Iowa on June 17 and is making a slew of stops around the country to convey concern about social injustice.

"Our bus is about a policy conversation ... to say we the people of the United States, we can be a better union," organiser Simone Campbell told AFP during a stop in Harrisburg, Pennsylvania where the sister on Thursday hailed a historic US Supreme Court ruling that upheld the constitutionality of President Barack Obama's signature healthcare law.

The idea for the trip, sponsored by the Catholic lobbying group Network headed by Campbell, came about in May after the Vatican released a report that charged the Leadership Conference of Women Religious (LCWR), which represents most US nuns, of "radical feminism" and not focusing enough on fighting gay marriage.

Network, which did not back Catholic bishops in their opposition to Obama's healthcare law, was taken to task by the Vatican for its links to LCWR.

"It was like a punch in the stomach," said Campbell, who was greeted with warm applause and autograph requests as she arrived in Harrisburg.

Describing herself as a strong woman and lawyer, Campbell, who is the executive director of Network, laughed off the notion that she was radical.

"It doesn't seem radical to me, it's not who I am – I am just a woman who cares passionately about people in poverty, that's it," she said.

Following the Vatican's rebuke, the sisters decided to ride the wave of media attention and draw attention to their cause.

When someone brought up the idea for a bus tour, they seized the opportunity and within 10 days had raised $150 000 for the cause.

Before long, they boarded a colourful bus that resembles one used by rock stars or politicians on the campaign trail.

Not to be missed is the huge inscription: Nuns on the bus coupled with their theme: ‘nuns drive for faith, family and fairness’.

At any given time, between four and seven sisters – dressed in regular clothing – take part in the tour. While others take shifts, the stalwarts are 67-year-old Campbell – an avid tweeter – and 81-year-old Diane Donoghue.

Since the start of the tour on June 17, they have passed through the states of Wisconsin, Illinois, Indiana, Michigan, Ohio and Pennsylvania. On Monday, with an eye on congressional offices, they are due in Washington.

On their various stops, Campbell, a registered lobbyist, denounces congressional budget cuts, highlighting how millions of people risk losing food stamps and access to Medicaid, US government health insurance for the poor.

"I work very hard to get federal policies that reflect our moral principles," she said. "I annoy the Democrats and I annoy the Republicans... the issue is caring for people at the margins."

The Catholic leaders have not commented on the tour, a fact Campbell -- who has defended the poor for four years – doesn't take too much to heart.

"Historically, nuns have always annoyed bishops," she said jokingly.

Others are certainly enthusiastic about the endeavour.

"They really restore your faith in the country," Mary McAndrews, a retired French teacher, told AFP as she attended a packed session organised by the nuns Thursday night in the chocolate-producing Pennsylvania town of Hershey.

But Bill Eakin feared the reaction of the church, saying it did not like "spontaneity".