The bus Sister Simone Campbell is using for her cross-country publicity tour is the type typically used by rock bands. To some, this seems appropriate. The D.C. nun was greeted in Jackson, Mich., with “Saint Simone” signs, and in Janesville, Wis., people inside a downtown office-building atrium lined the balconies chanting and snapping photos.

In the past couple of weeks, the dry-humored lobbyist has been on the “The Colbert Report.” “The Daily Show,” which will feature Campbell in July, made her a satiny, “Grease”-like jacket emblazoned with “Bad Habitz” on the back.

With the number of U.S. nuns plummeting in recent decades, many people have never seen one in person. Even fewer have seen a nun do something that appears as defiant as Campbell’s “Nuns on the Bus” tour, which rolls into the D.C. area this weekend in its full-size, advertisement-wrapped, spokeswoman-staffed bus.

The two-week trip, which began June 18, is an attempt to motivate opposition to a House budget that would sharply reduce spending on social services. But it is also a response of sorts to a Vatican report in April raising alarm about “radical feminism” among top American nuns and singling out Network, the D.C.-based social-justice lobbying group Campbell heads.

The report said many nun leaders are focusing too much on ­social-justice issues and too little on same-sex marriage and abortion. Further, the bishops said some of the ideas that liberal nuns such as Campbell are discussing publicly — among them, female priests — border on heresy.

You might think that such a critical papal spotlight would send all nuns deep underground. But that’s certainly not the case for Campbell, who is an unusual combination of hard-hitting political strategist and poetry-writing spiritual figure.

Instead, Campbell met with her political allies on the Hill and prayed hard. Then the light bulb came on: a road tour!

The tour’s unspoken, but nonetheless loud, message: The nuns’ moral compass is working just fine, thank you. “Their big mistake was naming us,” Campbell said. “With all this attention, we had to use it for our mission.”

Campbell’s tour also overlaps almost exactly with the two weeks the nation’s Catholic bishops have devoted to the idea that religious freedom is under assault in the United States. The bishops’ key target is the Obama administration’s health-care plan, which mandates contraception coverage and for which Campbell prominently lobbied.

Not a single bishop has taken Campbell up on her offer to meet since the tour began.

“Sister, you and your fellow nuns have clearly gone rogue!” Colbert deadpanned to Campbell, a petite woman who favors jangly earrings and theatrical eye-rolling. “I think your nuns should be intimidated a little bit more. The pope and the Vatican said, ‘Knock it off with the social liberalism.’ ”

The national attention being focused on a bus tooling across the Midwest with five nuns aboard says everything about the United States’ charged political climate. It wraps up gender, partisanship, religion and health care, all things that make a sexy story inside the Beltway these days.

It also shows the yearning among left-leaning Catholics for a prominent institutional voice. Decades ago, they had multiple heroes — Dorothy Day and the Berrigan brothers among them — who could champion issues involving war, poverty and racism, but today the church hierarchy is zoomed in on traditional marriage and abortion as the markers of Catholic identity. Even as polls show Catholic voters evenly split, it can still feel like a lonely time to be on the Catholic left.

“There aren’t enough clerical leaders who are sympathetic to stuff on the left who have the courage to step apart from their superiors,” said David J. O’Brien, a historian of the Catholic left. “Then there are women like Simone, who are so articulate and engaged.”

Campbell’s style of engagement on the Hill is part Zen master, part political tactician. One minute this is how she describes the tension between liberal nuns and church authorities: “I know we are one body. I know God hums my existence at every second, and yours, too.”

The next, she determines the Vatican report is payback for the nuns’ political effectiveness during the health-care debate. “They got bad political advice and are blaming us,” she snaps.

A legal aid lawyer for the poor in her home state of California for decades, Campbell was raised in a home in which being Catholic meant pragmatic action, said her sister, Toni Potter, a federal contractor who lives in Vienna.

“Talking about Catholic spirituality was not part of our DNA. It was like: ‘We’re Catholics, we get it, now let’s get something done,’ ” Potter said.

But Campbell worked to become more spiritual, she said, after the sisters in her order criticized her for being too focused on policy. In 2004, she became executive director of Network. Staffed exclusively by sisters when it was founded in 1972, Network is seen as having outsize impact for its modest, $800,000-a-year budget, in good part because of the moral weight of the sisters.

Campbell and Sister Carol Keehan, head of the huge Catholic Health Association, were considered key to the White House passing the health-care law; their approval helped balance bishops’ concern that the plan could provide federal funding for abortions.

“When that happened, many [Catholic] House members felt: ‘If the sisters feel that way, we shouldn’t be worried.’ And it wound up helping to break the deadlock,” said Ron Pollack, executive director of Families USA, a group advocating for affordable health care.

Campbell also bucked the bishops on their efforts to overturn a part of the new law that requires employers — including faith-based ones — to provide access to contraception coverage for their employees. Campbell “trusts the word of the administration” that the details will be worked out, her spokeswoman said.

Campbell’s critics say the bus tour is the epitome of disrespect.

The bus tour is “literally the only thing they have left: a dwindling group of hold-over pantsuit activists from the 1970’s,”conservative blogger Tom Peters wrote Tuesday as the bus tour rolled through Cleveland. “The Catholic Left: A Bus to Nowhere,” he titled his piece.

Campbell herself can be tart. She puts on the same whiny, mocking voice whether she’s making fun of herself or, for example, Rep. Paul Ryan, the Catholic congressman from Wisconsin who wrote the budget proposal that the tour targets.

“This nonsense about shifting more money to the top and devastating poor people — that’s wrong! And calling it Catholic social teaching in keeping with [his] conscience,” she said, putting on her drippy voice. “Bull! In keeping with his campaign donors.”