In battling for the hearts and votes of New Hampshire residents, Bill Bradley has just unleashed his latest weapon: Niki Tsongas.

"What disappoints me now is--just as with Paul--Bill's record is being distorted," the wife of the late Massachusetts senator says in a new ad. "But we don't have to listen to the distortions."

Bradley has made extensive use of surrogates in his advertising campaign. Nebraska Sen. Bob Kerrey, New York Sen. Daniel Patrick Moynihan and former Boston Celtics star Bill Russell have all pitched Bradley in 30-second spots.

Other White House contenders are following a similar path. Vice President Gore used Iowa Sen. Tom Harkin in two ads. George W. Bush has aired testimonials from New Hampshire Sen. Judd Gregg and Iowa Sen. Charles E. Grassley. John McCain has leaned heavily on two South Carolinians, Reps. Lindsey Graham and Mark Sanford, and used former New Hampshire senator Warren B. Rudman in a radio spot.

The question is: Do such ads change any minds?

"Endorsements matter because some people have such a high degree of credibility that voters find them very persuasive," said Darrell West, a Brown University political scientist. He described what he called the "cueing effect," in which voters take cues from local figures whom they trust.

But West doubted the Tsongas ad would be effective, saying, "She's not really a public figure anymore and her husband's campaign goes back eight years, which is an eternity in politics."

Kristen Ludecke, a Bradley spokeswoman, said that "Niki Tsongas is well-respected in New Hampshire. This ad is really about Al Gore. For the past several months, he's run a campaign of distortion."

Ludecke's comments underscore one benefit of such commercials: The surrogate can say harsher things about an opponent while the candidate--in this case Bradley--appears to stay on the high road. "The Bradley campaign has yet to defend any of our challenges on the merits," said Gore spokesman Chris Lehane. "Instead, they're just trying to distract voters."

Endorsement ads are hardly new; Eleanor Roosevelt made one for Adlai Stevenson in 1952. But they can be valuable, said Greg Stevens, McCain's media adviser, when a politician is as locally popular as Rep. Graham.

"Particularly in South Carolina, where John McCain simply isn't as well-known, we felt it would help move him along," Stevens said. "It isn't something that works all the time; it depends on who you're using as a surrogate. The mere fact that one politician endorses another isn't particularly potent these days."

Graham took off the gloves--saying McCain would "stop Bill Clinton's betrayal of our military"--in a way the Arizona senator would have been reluctant to do. Similarly, Harkin in essence accused Bradley of opposing flood relief for Midwest farmers, sparing Gore the chore of making the charge.

Another kind of third-party endorsement much beloved by candidates are newspaper editorials. Steve Forbes immediately aired a commercial trumpeting the Manchester Union Leader's support for his candidacy, and Bradley did the same with the Des Moines Register endorsement. McCain is considering an ad with supportive editorials by several New Hampshire papers.

"Despite all the hand-wringing about the political leanings of the media," said McCain spokesman Howard Opinsky, "a newspaper editorial bestows a certain degree of respectability on a candidate. People see it as an unbiased observation."