William "Mo" Cowan, Gov. Deval Patrick's former chief of staff and one-time chief legal counsel, has been chosen by Patrick to become the state's interim U.S. Senator temporarily replacing John Kerry, who was confirmed Tuesday as the country's next secretary of state, according to source in the governor's office. 

Cowan announced in November that he planned to resign as chief of staff this month to return to the private sector. Though he stepped aside the first week in January for new chief of staff Brendan Ryan to take over, Cowan has stayed on in the administration as a senior advisor to the governor, and planned to stay on through the month of January to help with the release and roll out of the governor's budget.

Patrick will formally announce Cowan at an 11 a.m. press conference in his office. Cowan, 43, lives in Stoughton with his wife and two sons, ages 8 and 4. He graduated from Duke University and Northeastern University Law School.

Since taking over for Arthur Bernard as chief of staff in January 2011, Cowan has helped lead the administration through a shakeup of the parole board following a controversy over the shooting death of a Woburn police officer and passage of a major reforms to the health care system currently being implemented.

Cowan will serve in the U.S. Senate alongside Sen. Elizabeth Warren until late June when a special election will be held to fill the remainder of Kerry's term through 2014. The governor on Wednesday is also expected to formally set the date of the special election for June 25, with party primaries to be held on April 30.

U.S. Rep. Edward Markey (D-Malden) is the only candidate in the race so far, but U.S. Rep. Stephen Lynch (D-South Boston) is expected to a nounce his intentions to run for the Senate on Thursday.

On the Republican side, former Sen. Scott Brown has been quite about whether he plans to run again for the third time since his surprise victory over Attorney General Martha Coakley in  2010.