Soon-to-be Secretary of State John Kerry’s formal bowing-out via resignation letter of the U.S. Senate seat he occupied for 29 years will officially trigger the special election to fill the vacancy in Massachusetts. After indicating that he was looking for an “out of the box” candidate who hasn’t run for political office before to act as interim senator until the June 25th election, Gov. Deval Patrick announced his official pick for the five-month placeholder on Wednesday, reports Politico:

Massachusetts Gov. Deval Patrick (D) will appoint his former chief of staff, Mo Cowan, to the Senate today, according to a Democrat with knowledge of the pick. …

In picking Cowan, Patrick passed over other candidates including former congressman Barney Frank (D-Mass.) and Vicki Kennedy, the widow of former senator Ted Kennedy (D-Mass.).

Cowan will become the second African-American to join the Senate in the last month, along with South Carolina Sen. Tim Scott (R), another appointee. When he is sworn in, the Senate will feature two black members for the first time ever.

As a former top adviser to Democratic Gov. Patrick, I think we can expect interim Sen. Cowan to generally hold the Democratic line in the Senate — but whether or not that seat will remain in Democratic hands is very much up in the air. Scott Brown has yet to publicly decide on whether he’ll wait for the Massachusetts gubernatorial race in 2014, or if he’ll once more enter into a special Senate election; it wouldn’t be his first rodeo, and early polling suggests he’d have a solid advantage going into it.

The Democrats will probably be duking it out in a primary between U.S. representatives Steven Lynch and Ed Markey — who’s already getting a head start on being the race’s resident ‘man of the people’ by calling for the same type of super PAC-pact agreed to by Brown and Elizabeth Warren in the November Senate election, via HuffPo:

Massachusetts Senate candidate Rep. Ed Markey (D-Mass.) wants to keep independent groups out of the special election he plans to run in should Sen. John Kerry (D-Mass.) vacate his seat to become the next Secretary of State.

On Monday, the Markey campaign released a challenge to all unannounced candidates to accept a “people’s pledge” to keep spending by independent groups out of the special election.

“I am challenging all of the candidates — Democrats and Republicans — in this special election for the United States Senate to join me in committing to the people’s pledge upon entering this race,” Markey said in a statement released by his campaign. “If all the candidates agree, we can give the voters the kind of debate they deserve. This election should be a forum for the voices of everyday voters, not attacks from Karl Rove and other outside special interests. This election should be focused on big issues and ideas, not big-money outside groups.”