(CNN) - Massachusetts Gov. Deval Patrick will announce an interim replacement Thursday for the U.S. Senate seat left open by the late Sen. Ted Kennedy, and hours before the announcement, two Democratic sources, one of whom is close the Kennedy family, tell CNN that Patrick has picked Paul Kirk.
Wednesday two Kennedy family associates told CNN that Kirk, a longtime senior aide to the late Sen. Kennedy and former chairman of the Democratic National Committee, is the family's favorite to fill his vacant Senate seat – and that the late senator's sons have relayed this view to Patrick. One source said the late senator's widow, Vicki Kennedy, was among the family members supporting a Kirk appointment.
The announcement is scheduled for 11 a.m. ET at the State House in Boston.
The appointment would give Democrats a crucial 60th vote in the Senate as the chamber weighs President Barack Obama's health care proposal.
The measure, which Wednesday afternoon passed the Massachusetts House and Senate,
would not go into effect for 90 days - a period that ends just a month before the scheduled special election for a permanent successor to complete the remainder of Kennedy's Senate term– unless two-thirds of the state House voted to bypass the delay and enact the measure immediately.
Democrats fell just short of that mark, with a final state House vote of 95-59. But Patrick also has the power to declare an emergency, which would allow the provision to go into effect right away.
Primaries to fill the seat for the remaining three years of Kennedy's six year term are set for December 8, while the general election will take place on January 19.
Republican State Sen. Scott Brown has announced that he will run for the seat on the Republican side, while state attorney Gen. Martha Coakley, Celtics co-owner Stephen Pagliuca and Rep. Mike Capuano are battling for the Democratic nomination.
–CNN's John King, Gloria Borger and Rebecca Sinderbrand contributed to this report