John Tierney, the eight-term congressman from the North Shore, survived his toughest challenge by far, beating a talented moderate Republican, Richard Tisei. After struggling to explain an offshore gambling scandal involving his wife and in-laws, Tierney clearly was vulnerable. His 1-point win can be ascribed to many factors — from the big Democratic turnout for President Obama to the presence of a third-party candidate who siphoned critical votes from Tisei. But also crucial were the accomplishments in Tierney’s record, including his work on important student-loan legislation and his significant contributions to the Affordable Care Act. If there’s a lesson in Tierney’s victory, it shouldn’t be that Democrats always win. It should be that Massachusetts voters will reward their representatives for concrete achievements. Tierney should take that lesson to heart as he seeks to put the scandal behind him.