The Dixie Chicks backlash hits us almost as soon as we touch down on American soil. Standing in line at Cincinnati airport, an immigration officer asks the purpose of our visit. When we tell him we're here to interview the Texan trio, he refrains from spitting on federal property. But you can hear the sound of phlegm gathering in the back of his throat. "They should string those girls up," he snarls.

Before we know it, we've been hauled off to have our bags searched and we miss our connecting flight to Memphis, where the Chicks are playing the following night. Welcome to America. When we belatedly arrive in Tennessee, several hours later, our taxi driver is similarly unimpressed with the 25m-selling one-time saviours of American country music. "Those girls need to learn to keep their mouths shut," he says.

Ever since Natalie Maines, the Dixie Chicks' lead singer, told a London audience they were "ashamed that the president of the United States of America is from Texas", the group has found itself in the eye of a storm that has threatened to destroy their careers. The comment, made last March during a concert at the Shepherd's Bush Empire, three days before America and Britain went to war in Iraq, was applauded by the audience.

The American ambassador to Britain didn't appear overly offended and, after the show, asked to have his photograph taken with the trio. But when a review of the concert in the Guardian, the only newspaper whose critic reported the comment, was picked up by a country-music website in Nashville, all hell broke lose. Before you could say shock and awe, Clear Channel, which owns 1,200 radio stations in America and helped to fund Bush's election campaign, had banned Dixie Chicks records from the airwaves "out of respect for our troops and our listeners". Cumulus Media, the second largest radio conglomerate, with 270 stations, also banned them, while right-wing press commentators had a field day denouncing them as traitors and dubbing the group "the Dixie Sluts" and "Saddam's Angels".

Six weeks earlier they had sung the national anthem at the Superbowl. Now, public CD-burning parties were being held all over the south and mid-west in scenes unprecedented since the Beatles' records suffered the same fate almost 40 years earlier, after John Lennon had suggested the group was bigger than Jesus. In one town in Louisiana, a steamroller crushed piles of offending Dixie Chicks records. Sales of their current album, Home, plummeted from 124,000 in the week the story broke to 33,000, an inevitable result of their banishment from the airwaves. Their number one country single, Travellin' Soldier, also went into freefall down the charts.

And there were more sinister aspects to the backlash. Death threats against Maines led to the need for 24-hour armed protection. Martie Maguire and Emily Robison, her fellow Chicks, stood by her in admirable solidarity and said that any one of them could easily have made the comment. So Robison's Texas ranch was trashed.

The obnoxiously gung-ho country singer Toby Keith (who, in the wake of 9/11, scored a country number one with an offensive record called Courtesy of the Red, White and Blue (The Angry American), which threatened "we'll put a boot in your ass, it's the American way", launched his own anti-Chicks crusade. His shows now feature a backdrop depicting Maines and Saddam Hussein as lovers. Another image has her face superimposed on the body of a toad.

In conservative country-music circles, Keith's crude attacks have gone uncriticised. But Maines faced a fresh backlash when she responded by turning up to a country music awards ceremony wearing a T-shirt bearing the slogan: "f.u.t.k."

Less than two weeks before Maines fateful comment, tickets went on sale for the Dixie Chicks' summer American tour. A staggering $49m was taken on day one, beating the previous record held by the Rolling Stones. Like every date on the tour, when we caught consecutive shows in Memphis and Atlanta, they were sold out, despite some fans having returned their tickets in protest.

At several venues, demonstrations have been staged. In Little Rock, Arkansas, a few days earlier, a local radio station handed out anti-Chicks T-shirts. More ludicrously, the American Red Cross turned down the group's offer of a $1m donation from the tour proceeds because Bush is one of its patrons.

In Memphis, the audience is divided. The merchandising counter is selling Free Natalie T-shirts and doing a brisk trade. The road crew also sport them, after the phrase was coined by a DJ in Austin, Texas (where they live), who had bravely come out in support. But some fans are highly critical. "Natalie made those statements later on after we bought the tickets, otherwise we wouldn't be here," complained Rod Good, 44, from Alexandria, Ohio. Connie and Steve Vaughan, both 53, from Atoka, Tennessee, turned up in matching American flag shirts. "We want her to know we support the troops," they said.

Yet the show - which is prefaced by Elvis Costello's version of (What's So Funny 'Bout) Peace, Love and Understanding - is well received, even when a song called Truth No 2 is accompanied by a video featuring Martin Luther King, Gandhi and Malcolm X and footage of civil-rights protests. The film also shows archive footage of Nazi book burning before it ends with shots of the destruction of Dixie Chicks records and the on-screen messages: "SEEK THE TRUTH" and "TOLERANCE". Throughout the show, Maines sports a "Dare To Be Free" T-shirt, and it is lost on no one that we're in the city where King was assassinated.

The following night we see the show again in Atlanta, Georgia, where neither of the city's two country radio stations has played a Dixie Chicks song since March. But when there are a handful of cat-calls from the audience, Maines responds feistily. "If you're booing that's OK, because we love freedom of expression," she tells them. "But just remember. We've got your 65 dollars."

Backstage before the show, the Chicks had been in similarly defiant mood. In their initial shock at the backlash, Maines issued a qualified apology to Bush in which she said that whoever holds the office of president "should be treated with respect". Since then, her resolve has hardened. As Trotsky once observed, the proletariat is radicalised by experience of the struggle. And while many have abused and reviled the Chicks, to others they have become a cause célèbre.

Bruce Spingsteen was one of the first to post a message of support on his website. Dolly Parton has also told her fans they should carry on buying Dixie Chicks records. More significantly, political support has grown since early July, when the group's English-born manager, Simon Renshaw, testified before a congressional committee looking into the future of the radio industry.

He revealed that his office had received death threats, and offered evidence that right-wing organisations had orchestrated the campaign. He complained that the group's rights under the first amendment had been abused, and that "artistic freedom, cultural enlightenment and political discourse" had been undermined.

Many agreed. One committee member, Barbara Boxer, a Democratic senator from California, likened the corporate radio ban to Nazi Germany and the McCarthyite witch-hunts of the 1950s and called it "a chilling message to people that they ought to shut up". Reclined over a chair in a room marked "CHICKS LIVING ROOM" and with armed security posted outside the door, Maines is astonishingly upbeat about her ordeal. "There was a point where I felt sick," she admits. "But now I feel proud and empowered. I'm glad it happened."

Two days after the row erupted, Maines asked her fellow group members if they were mad at her. "I said 'No, it really could have been any of us'," Maguire says. "We were about to go to war, and before we went on that night we talked about how silly we felt having to go out and entertain when our hearts were so heavy with what was about to happen." Maines concedes that they are entertainers, not politicians. "But that night it felt just too strange not to say anything. It would have been trite not to acknowledge it. To say something that was true and real but in a jokey manner was my way of dealing with it. And I'd rather it was a political reason that brought us down off the top of the charts than a musical one." "It's changed us all for the better," Robison adds.

Their solidarity is self-evidently genuine. They are so in tune on the issue that they frequently finish each other's sentences, and they screech "Oh my God!" in unison when told of our experience in the immigration line. "You're just lucky you weren't French," says Maines.

Their anger at Bush is now expressed in far stronger and more coherent terms than the original off-the-cuff comment. "We were told the official White House quote on our ordeal," Maines recalls. "I thought it was going to be something empowering about the first amendment and our rights as American citizens. I don't know why I thought such an educated thing could have come out of there. Instead it was, 'Their fans have spoken.'"

"Which makes your mind go back to the death threats and the trashing of Emily's ranch and the corporate banning," says Maguire. "So is the President condoning those things?" Robison demands.

"He was asked about the end of the war in Iraq, and he said, 'Freedom is a beautiful thing and these people now have a right to speak and we've given them that'," recalls Maines. "It was everything he should have said when he was asked about us."

One of the group's first public responses to the radio ban was to pose nude on the cover of Entertainment Weekly with slogans such as "TRAITORS" and "SADDAM'S ANGELS" superimposed on their bodies.

"It deserved a strong response from us and we felt it had to be in your face," Maguire says. "The magazine wanted us standing in front of the American flag in our jeans and smiling for the cover. And we thought no. We had to hit them over the head with it and expose the absurdity of the things we were being called. It's made me realise our country has not progressed as far as I thought we had. If this can happen to three white girls playing country music... "

Robison picks up the thread: "They've set this tone that they're not to be questioned and if you do then you are unpatriotic. That's somehow gotten into the American psyche and that's scary. If you can't question your government then you are just mindless followers."

· The Dixie Chicks play the Royal Albert Hall, London SW7 (020-7589 8212), on September 14 and 15.