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

In choosing Cowan, Patrick has tapped a close confidant and long-time advisor who has been at his side for major policy battles on Beacon Hill in recent years. As Cowan adjusts to the scene in Washington, he'll provide the Patrick administration another partner in Congress and an advocate for the priorities and values of the governor.

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 remained 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 rollout of the governor's budget.

Patrick will formally announce Cowan at an 11 a.m. press conference in his office. When Cowan takes the oath of office, he will become the first African-American senator from Massachusetts since Republican Edward Brooke held the position for two terms from 1967 to 1979 when he was defeated by the late Sen. Paul Tsongas.

Cowan, 43, lives in Stoughton with his wife and two sons, ages eight and four. A native of North Carolina, he graduated from Duke University and Northeastern University Law School. Before joining the administration in 2009, Cowan was a partner at Mintz Levin in Boston practicing civil litigation.

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.

Former Congressman Barney Frank, a veteran of scores of Congressional policy battles, had hoped to be named interim senator by Patrick. With Cowan partnering with Warren, Massachusetts will be exceptionally short on experience in the Senate in the coming months.

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

Senate President Therese Murray on Wednesday morning congratulated Cowan, and said she hoped one day he would run for elected office. Patrick has said his preference would be that the interim senator not run in the special election, but it was not immediately clear whether Cowan had agreed to that condition.

"Mo Cowan is the perfect choice. He is the consummate professional with a great background and a spotless reputation. Mo is cool under pressure, and he is liked and respected by everyone who knows him. In fact, he would have been a great candidate to run in the upcoming special election. As the interim senator, I am confident in his ability to carry our needs and concerns on Capitol Hill. I wish Mo the best of luck and hope that in the future he will consider running for an elected office," Murray said in a statement.

The governor's office also released statements of support for Cowan from UMass Boston Chancellor Keith Motley, Cape and Island District Attorney Michael O'Keefe, former Governor's Councilor Thomas Merrigan and Republican state Rep. Daniel Winslow.

"Mo personifies the dignity, intelligence and calm judgment that we hope are qualities of all members of the US Senate. He has the ability to work across the aisle, which is sorely needed in Washington, DC. Congratulations to Senator-designate Cowan and congratulations to all citizens of Massachusetts who will be served by him in the coming months," Winslow said in a statement.

Rep. George Peterson, a Grafton Republican and assistant minority leader, said called Patrick's pick of Cowan "fine."

"It's the governor's choice. He's made the decision. That's fine. It's going to be temporary," Peterson said. "We're probably going to be working on a resolution to say that you can't run for the office as well as stay out of any political activity for the upcoming seat."

The is the second time Patrick has had to appoint an interim senator during his six years in office. He tapped Paul Kirk in 2009 to take the place of the late Sen. Edward Kennedy following his death. Peterson said Kirk was "very active in the campaign."

As for the special election, Peterson said he expects Brown to enter the race. "I think Scott, probably, at least my sense is leaning to run," he said.