NEW YORK — Even while maintaining its status as one of the few musical acts that can still fill stadiums, U2 is struck by how quickly its world is changing — musically and politically.

Charismatic front man Bono, in a reflective mood as U2 closes the North American leg of its “360” tour, notes the different, more polarized atmosphere in the United States since the band performed its anthem, “City of Blinding Lights,” at President Barack Obama’s inauguration in January.

“I didn’t think it could come to this so quickly, after the joyous occasion of that election,” Bono says in an interview on board the band’s plane, as they jet to another stop on the tour. “I thought America was looking good. ... Things are getting a little rough now.”

Bono says he’s been in touch with Obama and is confident the president will deliver on promises made during the campaign, including the singer’s favorite issue: funding to fight AIDS in Africa. “The Obama administration is just getting going. (He) has promised to double aid over the next years, because even though (President George W.) Bush tripled it, ... the United States is still about half as what European countries give as a percentage, and I think he knows that’s not right.”

Meanwhile, Bono the rock star and the rest of U2 are struggling a bit themselves — as incongruous as that might seem for a band that will have performed to millions of people before its tour wraps overseas next year. (U2 ends its North American tour on Wednesday in Vancouver, British Columbia.)

Like other bands in the digital age, U2 is struggling to grab new listeners. Its members admit to frustration at the average album sales for its most recent release and wonder, as bassist Adam Clayton put it, whether the idea of an impassioned rock ‘n’ roll fan is becoming a thing of the past. (One experiment — U2 is broadcasting one of this weekend’s concerts in Los Angeles on YouTube.com.)

“The commercial challenges have to be confronted,” Clayton says during an interview backstage at TV comedy show “Saturday Night Live,” as awaits the band’s performance on the show’s season kickoff. “But I think, in a sense, the more interesting challenge is, ‘What is rock ‘n’ roll in this changing world?’ Because, to some extent, the concept of the music fan — the concept of the person who buys music and listens to music for the pleasure of music itself — is an outdated idea.”

The band’s latest CD, “No Line on the Horizon,” debuted at the top of the charts when it was released in March and has sold a respectable 1 million, according to Nielsen SoundScan. But the CD, which features more electronic music experimentation from U2, is the group’s lowest selling CD in more than a decade. It represents a marked drop from 2004’s “How to Dismantle an Atomic Bomb,” which has sold 3.2 million copies to date, and 2000’s 4.3 million seller, “All That You Can’t Leave Behind.”

“No Line” is also an album that hasn’t had that one signature hit.

U2’s last CD had “Vertigo,” which wasn’t a huge song on the pop charts, but became so ubiquitous thanks to Apple’s iPod commercial that it might as well have been a No. 1 smash.

The first single from “No Line” — the driving, upbeat “Get on Your Boots” — didn’t have a similar platform and didn’t crack Billboard’s top 30 singles pop chart. Meanwhile, “I’ll Go Crazy If I Don’t Go Crazy Tonight” was featured in a Blackberry ad as part of the new partnership between the mobile device and U2 but was not released as a single.

Songs from the new album are clearly resonating with die-hard fans. “Get on Your Boots” drew one of the more frenetic responses from the crowd during a recent concert near Boston, as did the anthemic show closer, “Moment of Surrender.” Yet the album hasn’t had the impact for which U2 had hoped.

While noting that signature U2 songs such as “Beautiful Day” and even “One” weren’t massive or immediate hits, Bono does acknowledge disappointment that the band didn’t quite “pull off the pop songs” with the new work.

“But we weren’t really in that mindset,” he says, “and we felt that the album was a kind of an almost extinct species, and we should approach it in totality and create a mood and a feeling, and a beginning, middle and an end. And I suppose we’ve made a work that is a bit challenging for people who have grown up on a diet of pop stars.”

Some would argue that the Irish rockers — Bono, Clayton, The Edge and Larry Mullen Jr. — remain pop’s biggest act. They are entering their fourth decade of music-making with a string of awards, from Grammys to Billboard to Golden Globes, tens of millions of records sold and a social impact that few musical acts can ever hope to achieve. Still, they find themselves in the same challenging position as most pop groups today, who must seek new ways to connect with music buyers in a declining industry and an increasingly fractious entertainment world.

“Music exists in an environment where people are multitasking, and I think that’s a very different environment,” says Clayton, who grew up appreciating jazz but realized “it was for people who took life a certain way, but it wasn’t part of the modern world for me.

“I worry that the world of rock ‘n’ roll that I grew up in is destined to end up that way.”

U2, of course, is hardly in danger of becoming a band that only gets heard in obscure clubs or on niche radio stations.

Its “360” tour is a massive undertaking that has the band performing in the center of stadiums, hence the “360” title. The production, which includes stages that take days to dismantle, has been one of the top grossing tours in the country since it kicked off in September, despite a price tag that runs upward of $250 (at least 10,000 tickets for $30 have been made available for every show).

And when the band played at Giants Stadium in East Rutherford, New Jersey, last month, it broke attendance records with a crowd of about 84,500 fans.

“In many ways, U2 has had such an enormous amount of success over the years we’re almost proof against that,” the band’s longtime manager, Paul McGuinness, says, talking about U2 and the music industry’s decline. “We’re still selling a lot of recorded music, but it’s a much smaller part of our business than playing live. This tour, by the time it’s finished, we would have played ... to roughly 6 million people.”

It is during live shows when U2 feels the most connection with its audience. Despite the stadium shows and the immense stage structure, the band insists that this time, the set up has created perhaps a greater intimacy with fans than the group has enjoyed in the past. They are literally surrounded by fans.

“The staging itself is something we’ve tried to do for a long, long time. The idea of playing 360 — it’s never been done successfully, ... where everybody gets good sound and good visuals, and we managed to achieve that, I think,” says Mullen, who, like the rest of his band mates, is affable and thoughtful as he talks about U2 backstage at “SNL.”

“The thing about U2 has always been its audience, and in this environment, I think the audience is so important, and the reaction is so important,” he says.

On tour, U2 can best gauge fan reaction to the new material. Last month at the cavernous Gillette Stadium near Boston, it was almost as frenzied and passionate as the reaction U2 gets for its classics. A roar came from the crowd as the band opened the show with “Magnificent,” and the energy kept building as U2 performed four more new songs, including “Get on Your Boots.”

“Judging by the reaction to the album, live, I feel like it has really connected,” The Edge says. “There’s a lot of records that make great first impressions. There might be one song that gets to be big on the radio, but they’re not albums that people ... play a lot.

“This is one that I gather from talking to people. ... Four months later, they’re saying, ‘I’m really getting into the album now.’”

U2 is still hustling to promote the CD. When it was released in March, the group did “Good Morning America” and an unprecedented five-night appearance on “Late Show With David Letterman.” More recently, U2 appeared on “SNL.”

“I love to see an outsized band like U2 behaving like they’re in the kindergarten and just doing what you do with your first album — taking it to the market, setting up your table, selling your wares, selling it out the street corners, giving out fliers,” says an animated Bono, breaking into a wide grin behind his trademark sunglasses. “I think selling out is when you stop believing enough in your music to put yourself out to explain it to people.”

U2’s Blackberry partnership includes an application that allows users to download the CD and photographs, liner notes and more.

Yet the band is also careful not to be too unwieldy when it comes to attempting new avenues to promote its music.

“We’re trying to do everything we can on that front without having to change what we’re about artistically: The music stays sacrosanct,” The Edge says. “We are much more focused on being the best than being the biggest.”

And that means perhaps making the kind of album that doesn’t guarantee hits but does guarantee surprises and new ideas, which “No Line” has delivered.

“The biggest danger for a band like U2 is accepting that you’ve reached a certain age, and, therefore, you can just actually sit back,” says Mullen.

“That’s not what we signed up to do. We want to make relevant, great music, and Bono has said numerous times, ‘One crap album and you’re out,’” he adds. “We’ve avoided it so far.”