There was a certain generic familiarity to the proceedings at the jam-packed city auditorium in Lowell, Mass., on Sunday night: With Tina Turner’s “Simply the Best” blaring and more than 2,500 Democrats screaming and stomping their feet in delight, Bill Clinton strolled onto the stage arm in arm with his party’s candidate for Congress. Like running mates at a national convention, they beamed and raised their clasped arms, soaking up the affection of the party faithful.

Lowell, an old textile mill town 25 miles northwest of Boston, anchors the 5th Congressional District, where a special election will be held in two weeks. It’s hardly surprising that Mr. Clinton, one of the great natural campaigners of his generation, would accept a Democratic candidate’s plea for help, especially in a district in the same media market as southern New Hampshire.

Hardly surprising, that is, until you consider the identity of that Democratic congressional candidate: She is Niki Tsongas, and 15 years ago her husband’s improbable presidential campaign was ravaged by inflammatory attacks from Bill Clinton.

Maybe you’ve forgotten the unlikely chord that Paul Tsongas—with his poll-defying adherence to a message of economic sacrifice and his dry, self-deprecating style—struck with his fellow countrymen in 1992, and just how dirty Mr. Clinton was willing to get to stop him.

Tsongas—who died five years later of complications from a bone marrow transplant at the age of 55—never quite forgave Mr. Clinton.

Only on the evening of the Democratic convention in the summer of 1992 did Tsongas offer his endorsement of Mr. Clinton, and it was a grudging and perfunctory one at that. Midway through Mr. Clinton’s first term, Tsongas called his old foe “a threat to my children” for his budget priorities and set about trying to recruit a challenger—either within the Democratic Party or through a third party movement—to Mr. Clinton for 1996. He found no takers, but even in his final interviews, in 1996, when it was clear his old foe would easily win a second term, Tsongas talked only of the hard budget choices the president refused to make.

He died the same weekend that Mr. Clinton was readying for his second Inaugural.

“I’m not sure that Paul, in his lifetime, ever got over it,” Bob Kerrey, one of the other Democrats who ran for president in 1992, said in an interview last week.

Not that anyone in Lowell on Sunday would have gotten that impression.

“Somewhere, Paul Tsongas is smiling down on us tonight,” Mr. Clinton said in his warm, soothing drawl. The crowd—and Mrs. Tsongas—applauded.

This is Niki Tsongas’ first foray on her own into electoral politics, and the finish line in her congressional race is just two weeks away, on Oct. 16. She’s run a community college in Lowell since her husband’s death, but it’s the lingering appeal of her family name that has driven her campaign.

Still, this is the first time since the 1992 presidential primaries that the Tsongas name has appeared on a ballot in Massachusetts, an eternity in politics. Mrs. Tsongas won the Democratic primary by just four points and there have been a few unnerving signs since then, like the recent poll that had her G.O.P. opponent within 10 points—a triumph in itself for a Republican in Massachusetts.

This being Massachusetts and 2007, Mrs. Tsongas should still win, and comfortably. But evidently, she doesn’t want to chance things: It was her decision to invite the man who spoiled her husband’s magic moment back in 1992.

“We reached out to them and he very quickly agreed to come,” Mrs. Tsongas told The Observer.

“I’m very appreciative of his willingness to do this,” she said. “He’s a busy man, as we know, and so I think it’s great that he wants to do this.”

And on one level, her decision is easy to understand. Mr. Clinton’s presence landed the Tsongas campaign on every major Boston television station Sunday night and Monday, to say nothing of the print coverage it netted, invaluable exposure in a race in which media exposure is at a premium. And Bill Clinton is political gold in Massachusetts, the perfect reinforcement for Mrs. Tsongas’ effort to run against the Bush administration. If there were any doubts about Mrs. Tsongas’ chances of winning, the Clinton visit probably puts them to rest.

But what about Paul?

Three nights before the 1992 Colorado primary—then one of three pivotal “junior Tuesday” contests sandwiched between lead-off New Hampshire and the Super Tuesday bloc of Southern states—the top two contenders faced each other on a debate stage in Denver. Bill Clinton warned that a Paul Tsongas presidency would mean “hundreds more” nuclear power plants, a powerful bit of fear mongering meant to sow distrust among Colorado’s eco-conscious Democrats toward Tsongas, an accomplished conservationist and the author of the Alaska Lands Act of 1980.

“That is a lie,” Tsongas shot back. “A lie. A lie.”

“No one can argue with you, Paul,” Mr. Clinton snapped. “You’re always perfect.”

“I’m not perfect,” Tsongas said. “Just honest.”

Tsongas fell inches short in Colorado, then watched in agony as Mr. Clinton blanketed the make-or-break state of Florida with damaging aspersions on Tsongas’ commitment to Medicare and Israel. “Cynical and unprincipled,” Tsongas cried. But he lost the state—and the nomination.

After their ’92 campaign, he essentially never spoke to Mr. Clinton again. And Mrs. Tsongas, besides appearing with him when he came to Lowell (at the invitation of then-Congressman Marty Meehan) as president seven years ago, hasn’t been in touch either.

Moreover, during the 2000 presidential primaries, Mrs. Tsongas cut a last-minute television ad in New Hampshire, where her husband was and is recalled fondly, for Bill Bradley, pointing out that his record was being “distorted” by Al Gore the same way her husband’s was by Bill Clinton. Back then, Dennis Kanin, a Tsongas family friend who managed Paul’s 1992 campaign, said of Mr. Clinton and Mr. Gore: “They characterize people with bold vision as risks to the American people. The motive is winning—winning at any cost. Bill Clinton and Al Gore are not the only people in politics who practice this, but it is unfortunate.”

Mr. Clinton’s rationale for attacking Tsongas in the first place was the claim that Tsongas actually attacked him first, by running an ad that criticized Mr. Clinton’s call for a middle-class tax cut, which Tsongas called pure pandering and utterly impractical in light of the country’s mounting deficits. Sure enough, three weeks after Mr. Clinton took office in 1993, he addressed the nation in prime time—to concede that, as much as he wanted to, the deficit would prevent him from offering the tax cuts he had championed.

“There should have been vindication in that (for Tsongas),” Mr. Kerrey said. “But I’m not sure there was.”

Didn’t that freighted history seem somewhat incongruous with the scene in Lowell on Sunday?

“That was then and this is now,” was about all Mrs. Tsongas would say on the subject.

Mr. Kerrey, who campaigned for Mr. Bradley with Mrs. Tsongas in New Hampshire in 2000, said political spouses often have a more balanced perspective when politics turns ugly.

“She didn’t like it, but she wasn’t consumed by it,” he said of Mrs. Tsongas.

As for Mr. Clinton, it would be easy to ascribe cynical motives to his eagerness to pitch in. Lowell is only 10 miles downriver from Nashua, N.H., and the Boston stations that covered his trip serve southern New Hampshire, where a majority of that state’s Democratic presidential primary voters live. Plus, Mrs. Tsongas’ endorsement is hardly a valueless commodity in New Hampshire. One of the three Tsongas daughters is already working for Barack Obama in New Hampshire. Perhaps Mr. Clinton’s pilgrimage to Lowell, then, will be enough to keep a Congresswoman Tsongas from following suit.

But maybe he found absolution too. Paul Tsongas got under his skin in 1992 like few politicians ever have, prompting several uncharacteristically sarcastic and biting outbursts from Mr. Clinton. Tsongas, who campaigned as a cancer survivor on a “journey of purpose,” eschewed the typical free-candy promises of politicians in favor of calls for economic sacrifice and politically poisonous reforms to costly entitlement programs. What Mr. Clinton saw, as he told reporters at the height of the campaign, was “a very clever politician.”

Long after that campaign ended, Tsongas’ crusade didn’t, and in the final four and a half years of his life he never offered Mr. Clinton the kind of public approval that his other defeated rivals all ultimately did.

But in Lowell on Sunday night, Mr. Clinton found that approval, when Niki Tsongas said that “in Bill Clinton’s last year in office, the federal government ran a $230 billion surplus. Eight years later … our national debt is approaching $10 trillion, and the Clinton surplus has become the Bush deficit of record proportions.”

Paul Tsongas ran for president against the Reagan-Bush legacy of a $4 trillion debt—an unconscionable abdication by one generation of its responsibility to the next, he argued. Tsongas died before the budget surpluses of Mr. Clinton’s second term were realized, and while Mr. Clinton never did tackle entitlement programs with the intensity Tsongas advocated, it sure seemed that on Sunday night Niki Tsongas was offering Bill Clinton the “job well done” that her husband didn’t live to provide on his own.

For his part, Mr. Kerrey—one of the leaders Tsongas actually hoped would oppose Mr. Clinton back in 1996—is satisfied that Tsongas would ultimately have come around on the Clinton presidency.

“It would have made Paul pretty happy to see Bill Clinton up there campaigning for Nikki,” Mr. Kerrey said. “It’s a moment when you can really say all the bad feelings from that campaign have ended.”