Cowan was selected by Gov. Deval Patrick (D-Mass.) to temporarily fill Kerry’s former seat. A special election will be held in June to decide who will serve Massachusetts on a longer-term basis. 

Rep. Ed MarkeyEdward (Ed) John MarkeyOvernight Tech: Zuckerberg breaks silence on Cambridge Analytica controversy | Senate passes sex trafficking bill | EU pushes new tax on tech | YouTube toughens rules on gun videos Dem senator responds to Zuckerberg: 'You need to come' testify Zuckerberg breaks silence on Cambridge Analytica crisis MORE (D-Mass.) has announced that he will run in the special election. Former Sen. Scott Brown (R-Mass.) has said he won’t run.

Kerry, who attended the swearing in, left the Senate to serve in President Obama’s cabinet as secretary of State. The Senate overwhelmingly confirmed Kerry for the position last week.

Members of the chamber applauded as Cowan signed the oath of office. Sen. Tim ScottTimothy (Tim) Eugene ScottSenators demand cyber deterrence strategy from Trump CNN's Bakari Sellers: Democratic leadership is 'old and stale' Politicians, faith leaders react to passing of Billy Graham MORE (R-S.C.), the only other African American currently serving in the Senate, was one of the first people to shake Cowan's hand after he was sworn in.