BOSTON (AP) — William "Mo" Cowan, a former top aide to Democratic Gov. Deval Patrick, was tapped by Patrick on Wednesday to serve as interim U.S. senator for Massachusetts until a special election is held to fill the seat left vacant by John Kerry's confirmation as the nation's new secretary of state.

Cowan, 43, became chief-of-staff to Patrick in 2010 after serving as chief legal counsel during the governor's first term. He stepped down as Patrick's top aide in December, though he remained on as a senior adviser through the filing last week of the governor's state budget request.

He'll serve until after the special election, scheduled for June 25. He said he humbled by the appointment.