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* William Cowan would be interim senator until June election
* Succeeds John Kerry who is stepping down to be secretary of state
* Patrick bypasses former Representative Barney Frank (Adds details on campaign)
By Scott Malone
BOSTON, Jan 30 (Reuters) - Massachusetts Governor Deval Patrick on Wednesday named his former chief of staff, William Cowan, to temporarily fill the U.S. Senate seat that John Kerry will vacate this week, the governor's office said in a statement.
A Democrat, Cowan would be the second African-American in the current Senate and would serve until a June 25 election in which Kerry's permanent replacement is selected. The governor scheduled a formal announcement at 11 a.m. ET (1600 GMT).
In choosing his former chief of staff, Patrick bypasses recently retired U.S. Congressman Barney Frank, who had publicly said he wanted the job.
Cowan's appointment maintains the Democrats' 53 seats in the Senate. Republicans hold 45 and there are 2 independents.
Kerry was approved by the Senate on Tuesday to succeed Hillary Clinton as secretary of state. Democrat Elizabeth Warren, elected in 2012, now becomes Massachusetts' senior senator.
So far only one person, Democratic Representative Edward Markey, has announced his candidacy.
However, fellow Democratic Congressman Stephen Lynch said early this month he is "giving serious consideration" to running for the seat Kerry had held since 1985. If Lynch enters the race, he would face Markey and any other rivals in an April 30 primary.
On the Republican side, all eyes are on former Senator Scott Brown, who has maintained a low profile since losing a re-election bid to Warren.
Brown stunned the state's liberal establishment in 2010 when he won a special election to fill the seat formerly held by the late Edward Kennedy, becoming Massachusetts's first Republican senator in three decades. (Editing by Jackie Frank)