Markey has kept a consistent but tenuous lead in the polls over his Republican rival Gabriel Gomez, fueling Democratic worries that the party will suffer an upset similar to the one that brought Republican Scott Brown to the Senate during the 2010 special election.
Clinton and Obama both campaigned for the 2010 Democratic candidate,
Martha Coakley, as Brown began to surge in the polls in the final weeks
of the campaign.
To prevent a repeat of 2010, Markey's campaign and national Democrats have rolled out a series of prominent party members to stump for the candidate, and the Democratic Senatorial Campaign Committee and Majority PAC recently went on air attacking Gomez.