State Senate majority leader Stanley Rosenberg, a liberal Democrat from Amherst, has assembled sufficient support within the chamber to succeed Therese Murray as Senate president. But the timing of Murray’s departure remains unknown, forging a power dynamic that longtime observers call unprecedented.
Rosenberg claimed victory Wednesday over Senate Ways and Means Committee chairman Stephen Brewer, who had received backing from the Senate’s more moderate members. In a statement, Brewer conceded, saying he expects Rosenberg “will do a fine job as Senate president when that time comes.”
Murray, a Plymouth Democrat, has said she will complete her Senate term, which runs through next year. She praised Rosenberg. Senate rules require that she leave the presidency by March 2015.
“The Senate has a very ambitious agenda for this legislative session, and we have a lot of work left to do,” she said in a statement Wednesday. “My focus is on addressing these priorities, and it is my intention to serve out my term.”
Rosenberg, who would become the state’s first openly gay legislative leader, acknowledged the timing of his victory, nearly a year and a half before he said he expects Murray to step aside, could stir questions.
“These processes are often puzzling,” he told the Globe. “This happened a lot faster and a lot earlier, I think, than any of us thought. But it takes on a life of its own and develops momentum.”
Senators described a campaign between Brewer and Rosenberg as virtually free of hostilities.
That is vastly different than the race between state representatives Robert A. DeLeo and John H. Rogers to succeed Salvatore F. DiMasi as House speaker in 2009. That contest left wounds in the House that have yet to heal.
“Not a bad word was said about either of us, in either direction,” Rosenberg said.
The geniality belies a more complex dynamic in the Senate, where members privately confess to confusion about the shifting centers of power. Several speculated about a bifurcated power structure, with Rosenberg and Murray loyalists conflicted about whom to follow until Murray steps down.
If Murray does complete her term, the vote on who should succeed her would not take place for more than 17 months, when the composition of the chamber could be altered.
“That is unprecedented, and I don’t know what the tensions will be,” said Thomas F. Birmingham, president of the state Senate from 1996 to 2002. “Members might have to question who really is the Senate president, so I was surprised at the timing of this.”
Brewer holds a seat of prominence atop the Senate’s budget-writing committee, which puts him in charge of shepherding members’ fiscal priorities.
In his statement, Brewer said: “Our caucus should remain united. The Senate is an institution that is larger than any one person.”
Rosenberg said that, with the Senate holding only informal sessions during August, he expected the chamber to run smoothly.
A Rosenberg presidency would give Western Massachusetts its first legislator to head either legislative branch since the 1970s, when David M. Bartley overlapped as House speaker with Maurice A. Donahue, the Senate president Both men represented Holyoke.
Murray and Brewer, who lives in Barre, represent two of the state’s most conservative districts. Rosenberg’s is among the state’s most liberal.
Rosenberg, 63, cinched the commitments that put him over the top during the past week, said senators familiar with the closely held vote-
collection process, expanding beyond his base of Senate liberals and other senators from the western part of the state.
Rosenberg, who served five years in the House before moving to the Senate in 1991, has been the Senate’s point man on two politically dicey issues in recent years. At Murray’s behest, he handled redistricting for the Senate after the 2010 Census and was chief author of the Senate gambling bill.
He vied for the presidency in 2002, when Birmingham’s departure left an opening that was eventually filled by Robert E. Travaglini.
Rosenberg has battled health problems, last year missing several months of work while being treated for squamous cell carcinoma. He lives in Amherst with his partner, Bryon Hefner. Rosenberg grew up in foster care in Revere and Malden and graduated from the University of Massachusetts at Amherst in 1977.