A new year and a new lineup for the City Council shows unanimous opposition to a proposal to site a 350-megawatt gas-fired power plant on Brockton’s south side. Councilor at Large Jass Stewart reiterated his stance against the $350 million plant during a keynote address to commemorate Martin Luther King Jr. at Massasoit Community College last week. All other current members of the council oppose it as well. Former longtime councilor Todd Petti, who opted out of another run last year, supported the effort, along with new Mayor Bill Carpenter, who wants to push it forward. Members of the grass-roots group Stop the Power said Wednesday that such support bodes well for efforts to block the plan proposed by Brockton Clean Energy. “The City Council is resoundingly against the plant, including all seven councilors we endorsed that all won election in November,’’ Stop the Power spokesman Justin Kane said in an e-mail.