The Senate Foreign Relations Committee unanimously approved Sen. John Kerry (D-Mass.) to be the next secretary of State earlier on Tuesday, setting up a vote on the Senate floor later in the day. Kerry's nomination later passed the Senate with overwhelming support.
Patrick will announce his appointment to Kerry's seat on Wednesday. Patrick's former chief of staff, William “Mo” Cowan, and Victoria Kennedy, widow of Sen. Edward Kennedy (D-Mass.), are considered frontrunners for the pick.
Patrick has set the special election primary date for April 30, and the general election for June 25.
Though Rep. Stephen Lynch (D-Mass.) insists he has not yet made up his mind about a run, sources say he is likely to jump into the race soon after Kerry steps down, setting Democrats up for an unwanted primary race. Lynch's flirtation with campaign highlights vulnerabilities some Democrats see in Rep. Edward Markey (D), the front-runner to win the Democratic nomination to succeed Kerry
No Republican has yet officially announced his or her intentions to run for the seat, but Republicans in Massachusetts expect Scott Brown (R) to run again, just a few months after he lost to Sen. Elizabeth Warren (D-Mass.)
Secretary of State Hillary Clinton has said her last day on the job will be Friday.
--This post was updated at 5:04 p.m.
Alexandra Jaffe contributed.