Maryland Gov. Martin O'Malley addresses members of the Maryland House of Delegates on the first day of the 2013 legislative session in Annapolis on Jan. 9. (Patrick Semansky/AP)

Maryland Gov. Martin O’Malley plans to announce Tuesday that he will put the full weight of his office behind repealing the death penalty, a move that could tip the balance on an issue that has sharply divided the legislature for years.

The governor has long opposed capital punishment, arguing that it is costly and an ineffective deterrent, but he has only once before sponsored a repeal bill, which fell short in 2009. With the arrival of several new lawmakers since then, and a sense of renewed momentum, O’Malley (D) is set to try again, according to several aides, who spoke on the condition of anonymity to more freely discuss the planned announcement.

O’Malley will make the announcement, the aides said, at a noon rally Tuesday in Annapolis with leaders of the NAACP and other civil rights activists, who have made repeal of Maryland’s death penalty a priority this year.

Though the governor’s decision does not guarantee passage, “it makes a huge difference,” said Sen. Brian E. Frosh (D-Montgomery), chairman of the Judicial Proceedings Committee, where repeal bills have stalled in past years, with members arguing that the death penalty should be available for the most egregious murder cases.

A repeal “can’t get done without the governor, and having his active engagement will be vital,” said Frosh, who favors the abolishment of capital punishment.

With O’Malley’s name on the bill, it is certain to get heightened media attention, as well as the focus of a team of lobbyists employed by the governor’s office and personal attention from O’Malley himself.

Maryland has not executed a prisoner since 2005, but perennial efforts to repeal the death penalty have fallen short. Meanwhile, other states have moved more decisively. Last year, Connecticut became the fifth state in five years to abolish capital punishment.

Opposition to the death penalty still carries some political risks, even for Democrats.

But at a time when O’Malley is contemplating a run for national office in 2016, passage of a repeal bill in the Free State — which has five prisoners on death row — could burnish his credentials as a progressive leader with the ability to get things done.

O’Malley is coming off several victories at the ballot box in November, including voter ratification of an O’Malley-sponsored law that legalizes same-sex marriage and another measure, known as the Dream Act, to extend in-state college tuition rates to illegal immigrants.

And in the current 90-day session, which started last week, O’Malley has already signaled he will push other initiatives that resonate with key groups in his party: a package of gun-control and other measures in response to the Connecticut shootings and a bid to provide incentives to spur the use of renewable wind-energy off the Atlantic coast.

In recent months, O’Malley has been coy when asked about his plans on the death penalty. The issue gained traction a couple of weeks ago when Senate President Thomas V. Mike Miller Jr. (D-Calvert) said he is willing to let the full body debate the bill, even if it lacks the vote to get out of committee.

Repeal bills — which would replace capital punishment with life in prison without the possibility of parole — have stalled each of the past several years in Frosh’s committee.

Pro-repeal activists say they are far more confident of their margin in the House of Delegates than in the Senate.

Last week, The Post identified 23 likely Senate votes for a repeal bill, one short of passage. But an additional four members have said they would consider supporting O’Malley-backed legislation.

Maryland has had an effective moratorium on executions since late 2006, when the Court of Appeals ruled that the state’s death penalty procedures had not been properly adopted, halting executions until new regulations were issued by the administration.

O’Malley’s administration issued new rules in 2009, but they were withdrawn after legislators raised objections. The rules have not been reissued.

In 2009, Miller allowed a repeal bill to reach the floor as a courtesy to O’Malley, even though it lacked a majority of votes in committee.

The full Senate balked at the measure, however. Instead, the chamber approved a bill that tightened evidentiary standards in capital cases. As a result, convicts are eligible for the death penalty in Maryland only in cases where there is DNA evidence, a videotaped confession or videotape of the crime.

Several lawmakers, including Sen. James Brochin (D-Baltimore County), said they continue to think the 2009 compromise should stand. Brochin said that he worries that if the death penalty is abolished, prosecutors will lose a valuable “bargaining chip” when negotiating plea deals.

But even Brochin acknowledged the chances of a repeal bill greatly increase if O’Malley is the sponsor.

“I think if his name is on the bill, he will get his desired result on the Senate floor,” Brochin said.

Sen. Christopher B. Shank (R-Washington), who also supports the death penalty, said that he would hope that lawmakers would not react differently to a repeal bill sponsored by the governor than they would to one sponsored by a fellow legislator.

“But on a practical level, when the governor puts his full faith and credit behind a bill, they pull out all the stops and do whatever they can to get votes,” Shank said.

Shank pointed to the history of same-sex marriage legislation in Maryland. In 2011, a bill fell short when sponsored by a group of legislators. Last year, O’Malley put his name on the bill and it was muscled through both chambers.

O’Malley doesn’t win them all, however. Last year, a bill that he sponsored to raise additional revenue for transportation projects went nowhere in the House or Senate, as members showed little interest in making their constituents pay more at the pump.