Bonnie Raitt is something of an anomaly in pop music. The folk, blues and roots-rock singer has never had a top 10 single or album; she hasn't even released a record in four years. Nonetheless she can easily sell out the Warner Theatre and annually fill up the lawn at Merriweather or Wolf Trap. She will do it again at Wolf Trap tonight.

Some of our MTV heroes should draw so well.

Several theories are offered to explain why the 36-year-old singer with the trademark mane of red hair is more successful at the concert box office than on the record charts. If her following is not especially broad, it's deep. Her fans are incredibly loyal and turn out for shows regardless of the ups and downs of her recording career or the current trends of the top 10.

That loyalty is inspired by Raitt's integrity. Raised by Quaker parents who dragged her to civil rights and disarmament marches, Raitt has consistently played political benefits, even when they fell out of fashion before their recent resurgence.

A veteran of the acoustic folk and blues circuit, Raitt has remained true to the roots of American music, even when it kept her off the radio.

Perhaps the best theory, though, is that Raitt cares the most about performing live. Her father is the Broadway star John Raitt, who made his living with touring musicals, and early on she absorbed the lesson that singing for a live audience is the heart of the music business.

"I grew up with a dad who traveled and performed live all the time," she explains, "and I got to like that life style. I enjoyed going to a lot of different places, and I liked the fact that he had his days free to do whatever he wanted. I liked that immediate feedback you get when you play live. I decided that was something I would like to do.

"So performing came first. Making records was just something to do so I could go out on the road. It was like doing your homework for the real thing. It's not as if I never sold any records. I never had a hit single, but six of my albums are almost gold, and the other two are way past gold. But that connection with a live audience is what it's all about."

Raitt's last album was 1982's "Green Lights," a move to the roots-rock sound of Rockpile and NRBQ (which contributed two songs to the album). She followed that up the next year with "Tongue in Groove," an album with the same band and sound. She even went out on tour that summer to support the record, but Warner Brothers never released it.

"We had, shall we say, protracted contractual negotiations," she comments wryly. "I didn't get involved; I just left it to the lawyers and stayed out on the road for three years. We finally hammered out our differences, which were not artistic but financial. Now I get all their eldest children."

What she did get was clearance to finally release "Tongue in Groove." Instead of putting it out as it was finished three years earlier, Raitt decided to go back into the studio with Little Feat veterans Bill Payne and George Massenberg to add some new songs.

So half of the new album, due in August, comes from the 1983 sessions and half from the 1986 sessions.

"I felt as if 'Tongue in Groove' came out because I went on tour and played all those songs," she says. "So when the contracts were straightened out, I asked to redo part of the album, because I wanted to do something new. I retitled the album 'Nine Lives,' which is kind of a play on words -- it's my ninth album, but it also suggests the image of 'How many times can you kill somebody and have them still come back?' "

Raitt comes back to Wolf Trap with her new band, Padlock. It includes two alumni of New Orleans' Neville Brothers Band (keyboardist Ivan Neville and bassist Hutch Hutchinson) and two long-time Raitt sidemen (guitarist Johnny Lee Schell and keyboardist Marty Grebb).

The new album, with songs by Ivan Neville, Eric Kaz and Toots & the Maytals, mixes white roots-rock and black rhythm and blues with an aggressive rhythm section.

"The main thrust of the album is snaky R&B," Raitt explains, "sort of like Tina Turner's 'What's Love Got to Do With It' -- not old R&B, but new R&B. Luckily, black and white radio aren't so separate now. You can hear people like Tina and the T-Birds making it with a mixture of rock and R&B, so now is a really good time for my new album to come out.

"It's ironic in retrospect, but when 'Green Lights' came out originally, Warner Brothers was worried that it sounded too raw and garage-rock for my fans. Now it turns out that John Cougar Mellencamp and Bryan Adams and their bands were big fans of that record, and that sound has become a lot more respectable."

Mellencamp invited Raitt to sing with him and his band at the first Farm Aid concert last fall, and Adams wrote Raitt's next single, "No Way to Treat a Lady," which is due in July. Two weeks ago, Raitt finally met Adams for the first time. One hour later she sang "No Way to Treat a Lady" with Adams and his band before 15,000 people during the second Amnesty International concert, at the L.A. Forum.

Raitt, a cofounder of Musicians United for Safe Energy in 1978, has always played for a lot of political benefit concerts. She sang on the antiapartheid "Sun City" record last fall and more recently has been performing near her home in California to stop offshore drilling and the forcible eviction of native American tribes.

"People should have a conscience," Raitt argues. "We're all in this together. A lot of people had it in the '60s, and maybe it's happening again. Some people have always had a conscience, but the key is reaching out to the large numbers of people and getting them involved. That's where music comes in.

"Sometimes message music has been too pedantic, and nobody wanted to listen to political songs. You remember some of those songs they used to play at demonstrations; they were pretty drippy. But people like Sting, Jackson Browne and the Clash are making political songs that you can dance to -- and that makes a big difference. That's what I'm trying to do."