An organization urging adoption of a $15-per-hour minimum wage voted Saturday to begin gathering signatures to get a Seattle City Charter amendment on the November ballot, a move it hopes will provide leverage with city officials.

The group, 15 Now, held its first national conference at Franklin High School, with more than 250 people voting on what direction the movement should take.

Members voted to begin gathering a target of 50,000 signatures in support of amending the city’s charter so it would require: big corporations to pay a $15 minimum wage effective Jan. 1, 2015, with no reduction in pay to compensate for tips or benefits; a three-year phase-in for small businesses and nonprofits; and a yearly raise tied to cost-of-living increases.

About 30,000 valid signatures would be needed to get the amendment on the ballot.

The organization plans to file the signatures — thus, triggering a vote — only if city officials fail to pass their own measure that meets with the organization’s approval. The decision of whether to file the signatures will be made at a conference in June.

“If the City Council capitulates to the demands of big business, we will have our backup ready,” Philip Locker told organization members Saturday.

Locker is a national organizer for the Socialist Alternative Party and was the campaign political director for City Councilmember Kshama Sawant, a leader of the push for increasing the minimum wage.

Seattle Mayor Ed Murray has voiced support for a $15-an-hour minimum wage, but on Thursday he announced that his income-inequality committee has failed to submit a proposal for such a measure, due to a lack of consensus on several issues.

15 Now is centered in Seattle, but wants to create momentum nationally for increasing the minimum wage.

Ken Armstrong: 206-464-3730 or karmstrong@seattletimes.com