Cowan, flanked by Sen. Elizabeth WarrenElizabeth Ann WarrenWarren: Trump is a 'racist bully' Poll: Oprah would outperform Warren, Harris against Trump in California Democrats continue to dismiss positive impacts of tax reform MORE (D-Mass.) Kerry, was sworn in by Vice President Biden. He is now the Senate's second black senator, and the only black Democratic senator.

He will serve until a special election to replace Kerry occurs in June. Democrats are likely to retain the seat, as Massachusetts Republicans have been unable to front a strong challenger since former Sen. Scott Brown (R-Mass.) decided against a run.

Reps. Edward MarkeyEdward (Ed) John MarkeyDemocrats search for 51st net neutrality vote Overnight Regulation: Dems claim 50 votes in Senate to block net neutrality repeal | Consumer bureau takes first step to revising payday lending rule | Trump wants to loosen rules on bank loans | Pentagon, FDA to speed up military drug approvals Dems say they have 50 votes in Senate to overrule net neutrality repeal MORE and Stephen Lynch have both announced their intentions to pursue the Democratic nomination for the seat, and face a primary on April 30.