This week I met a nun from Philadelphia who works with immigrants.

Sister Eileen Marnien told me how the Sisters of St Joseph bought an old funeral home in her city and converted it into a welcome centre after a number of Catholic schools in the area closed down, leaving them with no work.

"I feel like we sisters do what we have always done," she said. "If there's a need, we step up."

Sister Eileen was attending a rally for immigration reform outside St Joseph's Pro-Cathedral in Camden, New Jersey, one of dozens of events organised by a group of sisters who call themselves the "Nuns on the Bus."

They've launched a 15-state bus tour to raise support for an immigration reform bill currently making its way through the US Congress.

"We have a very narrow window of time in which to get it done so we thought, what else can we do?" said Sister Simone Campbell, the head of the Catholic lobbying organisation Network and the force behind the Nuns on the Bus.

"It is the urgency of now that got us back on the road."

Last summer, the nuns had their first tour to protest proposed Republican budget cuts. The sisters got more attention, however, for getting in trouble with the Vatican.

Network and the Leadership Conference of Women Religious, an umbrella group that represents most nuns in the country, were criticised for being too outspoken on some issues and not deferential enough to the church hierarchy on others.

This time around, the nuns are in lockstep with the US Conference of Catholic Bishops on immigration. But that hasn't stopped the criticism or the controversy, highlighting divisions within the church on the role of women.

Senior Simone told me she wanted this tour to be about unity and so had invited bishops to take part in the rallies. They haven't had a lot of takers, however, and one of the few who was supposed to show up in Scranton, Pennsylvania dropped out at the last minute.

'Disloyal daughters'

Bill Donohue, head of the Catholic League, has been openly critical of what he likes to call "the magical mystery tour."

He hasn't forgiven the sisters for supporting Barack Obama's healthcare reform, which the bishops opposed due to a requirement that employers provide insurance coverage for contraception, which is against church teaching.

"The whole organisation is a fraud," Donohue said, when I asked him why he opposed the nuns efforts to further the church's position on immigration.

"It is run by people who are disloyal daughters of the Catholic Church. They will never speak out against abortion, they are supporting a pro-abortion administration."

He also said he had more respect for sisters who wear the traditional habit than these "polyester nuns".

But the nuns do have supporters within the church, including Monsignor Kevin Sullivan, the head of Catholic Charities New York office, who spoke at the tour's kick-off.

"In our catholic church there are many ways to exercise leadership," he said from the podium. "None have exercised greater leadership of service and advocacy than women religious."

Nuns like Sister Eileen have seen the effects of the country's broken immigration system: undocumented workers living in fear, parents deported and families torn apart. She says the people she works with have "amazing work ethic" and the poorest tend to be the most generous.

"We've forgotten how to be inclusive," she said, explaining why she had shown up at the rally. "We need to get back to our roots as Americans."

The bus that will carry the sisters along the southern border of the US over the next few weeks calls on Americans to raise their hands and voices for immigration reform by texting "nuns" to 877 877. Those who do will get a call the next day, putting them in touch with their political representatives.

The 25 or so nuns who will take turns riding the bus are also blogging and tweeting – and shrugging off their critics.