Markey received 55 per cent of the votes with 99 per cent of the ballots counted, said NECN. He will replace former Senator John Kerry, who was appointed as Secretary of State by President Barack Obama earlier this year.
Back in April Gomez, pictured, defeated ex-US Attorney Michael Sullivan in the Republican primaries after quitting his job at private equity firm Advent International to focus on the race.
While Massachusetts is a reliable Democratic state, Republican Mitt Romney won a term as governor in 2002 and another Republican Scott Brown was successful in a special election for the US Senate three years ago.
Advent has raised $37bn including the $10.8bn GPE VII closed in november 2012 and participated in more than 250 deals since its launch in 1984.
The firm targets companies in the business and financial services, healthcare, industrial, retail and technology, media and telecoms sectors.
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