HARTFORD — Early this morning, Connecticut took a critical step toward making the state’s death penalty a thing of the past.

After nearly 12 hours of debate in a session that began Wednesday afternoon, the state Senate passed a bill that would eliminate capital punishment in favor of life in prison without parole by a vote of 20 to 16. The measure now heads to the House of Representatives, where it is expected to pass within the next few weeks. Gov. Dannel Malloy has said he would sign the bill.

Senate Democrats, whose amendment toughening sentences for certain murders helped corral support for repeal, hailed the vote as the beginning of the end for an unjust policy.

“The death penalty, as a punishment, is deeply flawed in many ways,” said Sen.Eric Coleman, a Democrat from Bloomfield and the Senate chair of the Judiciary Committee, which approved the bill March 21. “It is a poor deterrent to crime, inequitably applied across racial and economic lines and a fundamental contradiction — killing as a punishment for killing.”

As the debate began Wednesday afternoon, much of the controversy centered on the fate of the 11 prisoners currently on death row in Connecticut. Democratic proponents of the bill said the measure is entirely “proactive,” meaning that it applies only to future convictions and will not repeal established capital sentences.

But Republican opponents to the measure were unconvinced, arguing that the measure would provide new ground for defense attorneys to appeal these existing sentences.

Susan Story, Connecticut’s chief public defender, advocated a retroactive ban on the death penalty in testimony during a March 14 hearing on the bill. She also indicated that her office might appeal prior capital sentences.

But Democrats said the experience of New Mexico should allay those concerns. In 2009, the New Mexico Supreme Court denied the appeal of an inmate with a previous capital sentence after the state passed a “proactive” death penalty repeal.

Sen. John Kissel, Republican of Enfield, the ranking Republican on the Judiciary Committee, was unconvinced by the New Mexico example, arguing that since it was not clear whether the ruling was constitutional, there remained room for a different judicial interpretation.

At the Monday morning press conference, Senate Democrats also proposed an amendment to the repeal bill that creates a new set of punishments for criminals convicted of “felony murder with special circumstances.” They said they designed this category specifically for the purpose of replacing the capital sentence.

“These inmates will face conditions that are similar to and in some cases more severe than conditions on death row,” Senate President Donald Williams, a Democrat from Brooklyn, Conn., said. “It is a punishment and sentence that is certain and final.”

Williams also said it was “one of the key factors” in getting other lawmakers to sign on the bill.

The penalties include high-security housing, continuous escort and increased searches, among others. These conditions would apply for the duration of a life sentence for those convicted of the new felony category.

The amendment passed 21–14 on a vote that went entirely along party lines.

Kissel criticized the amendment’s inclusion of a risk assessment of these inmates, which he argued will allow for too much flexibility in how the additional penalties are applied.

“We are spelling out in statute that [the Department of Correction] will do a risk assessment,” he said. “There is no guarantee that future heinous, diabolical criminals will end up in housing like this.”

He offered an amendment that removed the possibility of a risk assessment, but it failed along the same 21–14 party-line vote.

This is lawmakers’ third attempt to repeal the state’s death penalty since 2009. A repeal passed both houses of the General Assembly in 2009, only to be vetoed by former Republican Gov. Jodi Rell. A similar effort passed the Senate Judiciary Committee in 2011, but it never came to a vote after two lawmakers withdrew their support, citing the ongoing trial over the infamous home invasion and triple homicide in Cheshire, Conn.

A March 21 Quinnipiac University poll found that 62 percent of Connecticut residents consider repealing the death penalty a “bad idea.” But at the press conference, Williams said the wording of such polls often influences the result, adding that respondents tend to favor repealing the death penalty when life without parole is given as the alternative. In an earlier Quinnipiac University poll, conducted in March 2011, 49 percent of respondents favored life without parole over the death penalty when given the choice between the two.

Connecticut has executed one individual since the 1976 Supreme Court case Gregg v. Georgia lifted the ban on capital punishment.