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 today 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 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.
At a news conference today, Cowan said he is "not a candidate for public office."