The state House of Representatives voted 247-123 last week to override Sununu's veto, just reaching the required two-thirds threshold. On Thursday, 12 Democrats and four Republicans supported ending the death penalty while six Republicans and two Democrats voted to keep it, once again reaching the two-thirds mark by the narrowest possible margin.
"It's up to us to stop this practice that is archaic, costly, discriminatory and final," said Democratic state Sen. Melanie Levesque.
New Hampshire hasn't executed anyone since 1939 and the state's death penalty only applied to seven scenarios: the killing of an on-duty law enforcement officer or judge, murder for hire, murder during a rape, certain drug offenses, or home invasion and murder by someone already serving a life sentence without parole.
There is currently one prisoner on death row in New Hampshire: Michael Addison, who is awaiting execution for the 2006 killing of Manchester Police Officer Michael Briggs. Thursday's vote does not apply retroactively, but death penalty supporters argued that courts would interpret the matter differently.
"If you think you're passing this today and Mr. Addison is still going to remain on death row, you are confused," said Republican state Sen. Sharon Carson. "Mr. Addison's sentence will be converted to life in prison."
Democratic State Sen. Lou D'Allesandro, who represents the district where Briggs was killed, voted to keep the death penalty, invoking the dangers law enforcement officers face every day.
"I can't abandon these people," he said. "These people are there for us. They're there for us, and I believe strongly we have to support them."
Kelly Ayotte, a former U.S. senator and New Hampshire attorney general who prosecuted Addison for Briggs' murder, said she was disappointed with state lawmakers.
"Very disappointed and angry that the NH senate [sic] failed to sustain the Governor’s veto of the death penalty repeal," she wrote on Twitter. "Police killer Michael Addison is the happiest about their vote today."
Sununu, who vetoed the repeal bill surrounded by officers at a community center named for Briggs, said Thursday he was incredibly disappointed in the vote.
"I have consistently stood with law enforcement, families of crime victims and advocates for justice in opposing a repeal of the death penalty because it is the right thing to do," he said in a statement.
But Republican state Sen. Bob Giuda, a former FBI agent, said while he greatly respects law enforcement, the death penalty is at odds with his anti-abortion principles. He called execution a "ghastly" process and urged his colleagues to "move our civilization" past it.
"I think we're better than that," he said. "I choose to move our state forward to remove the death penalty."
The Associated Press contributed to this report.