John F. Kerry's only non-presidential loss in his 40+ Carreerin politics, his 1972 Race is one of the few Blemishes too his name, but despite loosing by 10.73% to a Republican incumbent Kerry proved that one loss doesn't make the man, but how you bounce back from it.
The main obstacle present to Kerry was the Local paper The Lowel Sun which went on an all out crusade against Kerry, effectively attacking him for carpet bagging and receiving out of state contributions.
Later on, some of the Watergate Tapes showed that Kerry's candidacy attracted the personal attention of President Richard Nixon, who took great interest in defeating Kerry. Going so far as to send operatives to defeat Kerry's campaign.
Despite being favored as late as a week before, the culmination of actions, such as the attacks & the Consolidation of the anti-Kerry vote with the Independent candidate dropping out and endorsing the Republican led to Kerry's defeat.
Kerry would later make a comeback a decade later, winning the Liutennant gubernatorial primary, and since this congressional race, John F. Kerry Never lost a single race in Massachusetts
An Addendum: George McGovern Won this district in 1972 with 53% of the Vote, to Richard Nixon's 47% of the Vote.