Mass. Gov. Patrick names ex-aide William 'Mo' Cowan to fill John Kerry's seat on interim basis 30 Jan 2013 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.