ANNAPOLIS, Md. - Opponents of capital punishment marked a milestone Thursday as Maryland became the first state south of the Mason-Dixon Line to abolish the death penalty.

The passage was a significant victory for Democratic Gov. Martin O'Malley, a Roman Catholic who opposes capital punishment and is considering seeking the 2016 presidential nomination. Death penalty opponents said the governor helped maintain the national momentum of repeal efforts by making Maryland the sixth state in as many years to abolish capital punishment.

"I don't know exactly what the timing is, but over the longer arc of history I think you'll see more and more states repeal the death penalty," O'Malley said in a brief interview after the bill signing. "It's wasteful. It's ineffective. It doesn't work to reduce violent crime."

NAACP President and CEO Ben Jealous, who worked to get the repeal bill passed, noted the significance of a Democratic governor south of the Mason-Dixon Line with presidential aspirations leading an effort to ban capital punishment.

Jealous noted that in 1992, then-Arkansas Gov. Bill Clinton left the presidential campaign trail to oversee the execution of a man who had killed a police officer, a move widely viewed as an effort to shed the Democratic Party's image as soft on crime.

"Our governor has also just redefined what it means to have a political future in this country," Jealous said. "You know, it was just 20 years ago that a young governor with possibilities below the Mason-Dixon stopped during his presidential campaign" to oversee an execution.

Maryland is the 18th state to abolish the death penalty. Neighboring Delaware also made a push to repeal it this year, but the bill has stalled.

State Sen. Jamie Raskin, a Democrat and constitutional law professor who opposes the death penalty, said he believes pressure is building around the country to focus law enforcement resources on things that are proven to lower the homicide rate.

"The trend lines are clear," Raskin said. "There's nobody who's adding the death penalty to their state laws. Everybody is taking it away."

The bill will not apply to the five men the state has on death row, but the governor can commute their sentences to life without parole. O'Malley has said he will consider them on a case-by-case basis.

The state's last execution was in 2005, when Republican Gov. Robert Ehrlich was in office.