TRENTON A top state lawmaker will seek to override Gov. Chris Christie's veto of legislation that would keep New Jersey in a multistate pact to reduce greenhouse gas emissions blamed for global warming. Assemblyman John McKeon, an Essex County Democrat who chairs the chamber's environmental committee, calls the governor's veto "a giant step backward" toward a clean energy economy. Christie vetoed the bill late Friday, affirming an earlier decision to pull out of the greenhouse gas reduction pact known as RGGI. Christie had announced in May that New Jersey would pull out of the program by year's end, saying the program was ineffective at stemming carbon dioxide pollution. Ten states are part of the pact to reduce carbon dioxide emissions in the Northeast by 10 percent by 2018.