Former Democratic Party chairman Paul G Kirk Jr. was sworn in on Friday as an interim U.S. Senator of Massachusetts, replacing the late Senator Edward M. Kennedy.
Vice-President Joe Biden swore in Kirk on Friday afternoon, hours after a Massachusetts judge rejected a request from state Republicans to delay the appointment.
The GOP had argued Mass. Gov. Deval Patrick had exceeded his authority in appointing Kirk before a traditional 90-day legislative waiting period had passed.
A bill giving Patrick the power to appoint a temporary Senator had already been approved in the state legislature and senate.
Kirk gives Senate Democrats a filibuster-proof, 60-vote majority the party needs to help push through Barack Obama's expected health-care bill.
Kirk has said he will serve as interim senator until voters choose a replacement in a Jan. 19 special election.