SEATTLE — Mayor Ed Murray presented on Thursday what he described as an imperfect but workable plan to increase the city’s minimum wage to $15 an hour, more than twice the federal minimum wage and one of the highest anywhere in the nation, through a series of complex and phased-in stages. Just as crucially, he said, the plan has broad political support, with a coalition of labor and business groups ready to push hard for it at the City Council, starting with the first hearings next week.

But the plan, which in many other cities might be seen as a liberal Democratic agenda at the frontier of social and economic engineering, was immediately attacked not from the mayor’s right, but from his left.

Kshama Sawant, a Socialist Alternative Party member who was elected to the Seattle City Council last year on a single-minded drive to raise wages, said the plan had been “watered down” by business interests on the mayor’s 24-member committee on income inequality, of which she was also a member. In a packed news conference at City Hall right after Mr. Murray’s, she called on her supporters to continue their effort to gather signatures for a possible ballot initiative on wages this fall. The campaign might also put pressure on the Council to make the mayor’s plan better for workers, she suggested. “Every year of a phase-in means yet another year in poverty for a worker,” Ms. Sawant said. “Our work is far from done.”

Mr. Murray, a Democrat and former state senator, formed his special committee on income inequality this year — headed by a labor union leader and a business executive — and gave them three months to find common ground. While running for mayor last fall, he, like Ms. Sawant, pledged to support a $15 minimum wage.