Democrats have enjoyed a long run of one-party hegemony on Beacon Hill, but on Wednesday their two leading candidates for governor found themselves a bit out of tune.

Both state Treasurer Steven Grossman and Attorney General Martha Coakley attended a morning fund-raiser for Senate majority leader Stanley C. Rosenberg, whose likely ascension to the chamber’s presidency ensured a healthy attendance.

Grossman arrived first, but could only look on as Coakley was ushered to the head of the receiving line waiting for Rosenberg, according to multiple lobbyists who attended the feting at Carrie Nation, a Beacon Hill restaurant that opened earlier this year.

Advertisement - Continue Reading Below

Grossman had a rejoinder. Summoning the attention of a startled crowd of lobbyists and other Rosenberg contributors, Grossman announced he would lead a rendition of “Happy Birthday” in honor of the Amherst Democrat, who had turned 64 the day before.

But Grossman is evidently a better politician than bandleader. Only a meager collection of voices joined in the song, according to several people who were present.

One lobbyist compared Grossman’s attempt to Red Sox chairman Tom Werner’s ill-fated effort to start a “Let’s go, Red Sox” chant at Fenway Park prior to this month’s World Series victory parade, an incident roundly mocked on sports talk radio.

“It was pretty awkward,” said another lobbyist who was present.

But Rosenberg can get used to people going out of their way, or off-key, to win his affection. He has claimed sufficient support among Senate Democrats to ensure he will succeed Senate President Therese Murray when she steps down.