After contentious mid-summer debates followed by four months of closed-door negotiations, House and Senate Democrats on Beacon Hill reached a compromise Monday on a landmark police reform and racial justice legislation.

The 129-page bill is based on legislation that passed through the House and Senate as a response to national protests against police violence and the disproportionate impacts communities of color experience from the criminal justice system.

The compromise creates a police accountability and oversight system, under which officers would need to be certified every three years and could lose their certification for violations, including excessive use of force.

“This is a landmark decision that begins to address the inequities that we have seen in our police institutions for a long time,” said Rep. Carlos Gonzalez, who chairs the Black and Latino Legislative Caucus and was one of the six conference committee members who negotiated the final version.

“Like everybody has said, one bill is not going to address every issue, but we are confident that this bill starts to begin the process of holding police accountable in a transparent way and having an independent body investigate police misconduct when and if it occurs.”

The conference committee’s bill was expected to surface for final approval on Tuesday, when the House and Senate are each scheduled to meet. Republicans appear likely to oppose the compromise after the two GOP negotiators, Rep. Timothy Whelan of Brewster and Senate Minority Leader Bruce Tarr of Gloucester, declined to sign the conference committee report.

All four Democratic conferees — Gonzalez, Rep. Claire Cronin, and Sens. William Brownsberger and Sonia Chang-Diaz — signed the jacket indicating their support for the legislation.

On Tuesday, Gov. Charlie Baker gave no signals at an early afternoon press conference about how that bill would be received if reaches his desk by the end of the day.

“I’m glad the Legislature moved forward on this. I’m glad that this was something that was part of what they considered to be important to get done before the end of the session. But I can’t speak to the specifics of this until we have a chance to review it,” Baker said.

Police union officials were quick to denounce the bill. The state’s largest law enforcement union called it the “final attack” by lawmakers on Bay State police, who union leaders said are being scapegoated for violence incidents that occurred in other states.

In a letter to his 4,300 members, Massachusetts Coalition of Police President Scott Hovsepian said the final policing bill that emerged from a House and Senate conference committee Monday evening was “crowded” with punitive measures for police and would add layers of unnecessary bureaucracy.

The union urged the 66 lawmakers who voted against earlier versions of the bill to oppose this compromise version as well, and to lobby their colleagues to join them. Hovsepian said that if MassCOP loses to the “iron hand” of leadership in the House and Senate it would appeal to Gov. Charlie Baker for a veto.

“They seek to punish police just for being police,” Hovsepian wrote in a two-page letter to union members. MassCOP joins the union representing State Police troopers in opposing the bill.

“The State Police Association of Massachusetts welcomes reform that will actually improve policing; unfortunately, this legislation misses the mark,” the union said in a statement issued late Monday. “The bill creates layers of unnecessary bureaucracy and costly commissions staffed by political appointees with no real-world experience in policing and the dangers officers face every day. We urge the members of the Legislature to reject this bill and begin anew in 2021.”

Passage of the original House and Senate bills in July was a contentious process, featuring multiple days of debate and vote margins — 93-66 in the House and 30-7 in the Senate — significantly tighter than most.

Gov. Baker filed his own, somewhat narrower police accountability and certification bill in June. He has not publicly taken a position on some key measures of the Legislature’s version, including its reforms of qualified immunity.

However, he may have more sway over what ultimately becomes law than he does on many other issues. While Democrats wield super-majorities in both chambers, the original House vote did not surpass a two-thirds majority, so legislative leaders may not have the numbers to pass something Baker does not like.

“I think it’s a great bill and I think the governor’s going to like it,” Brownsberger said. “There’s a lot in this bill that goes back to things the governor was thinking about, so I think [he] should be pleased with this bill.”

The legislation would create a Peace Officer Standards and Training Commission responsible for setting police standards across the state, certifying law enforcement officers, and investigating potential wrongdoing by police. Nine members would sit on the commission, with three hailing from law enforcement backgrounds and six coming from civilian life.

The governor would appoint three members, consisting of one police chief, one retired Superior Court justice, and one social worker nominated by the National Association of Social Workers’s Massachusetts chapter. The attorney general would also appoint three members, including one police officer below the rank of sergeant, one officer nominated by the Massachusetts Association of Minority Law Enforcement Officers, and one attorney nominated by the Massachusetts Bar Association’s civil rights and social justice section council.

The governor and attorney general would jointly appoint the remaining three members, one of whom must be nominated by the Massachusetts Commission Against Discrimination.

“There’s no other supervisory board in the country like this one with the extent of power that it has to monitor, to directly discipline and to hear complaints directly,” Brownsberger, a Belmont Democrat, said. “It’s a majority civilian board. Almost all of the rest of the boards in the country are all law enforcement, so this is a very, very strong innovation.”

The commission is empowered to examine alleged misconduct and revoke officers’ certifications for a range of offenses, including use of chokeholds, falsifying timesheets, and failing to intervene to prevent another officer from an excessive use of force.

On qualified immunity, a controversial policy that shields police and other public employees from facing civil lawsuits in state court, the conference committee’s bill ties the protections to the licensing procedure and revokes it in any case where an officer is decertified.

The bill also creates a legislative commission to study qualified immunity and report on its impacts as well as potential reforms by Sept. 30, 2021.

Negotiators embraced the approach in the House’s original bill and dropped the Senate’s language, which would have allowed lawsuits to proceed in some cases against officers who should have reasonably known they were violating civil rights.

“It landed on the House position,” Brownsberger said of the qualified immunity talks.

Asked for her thoughts on the final position, Chang-Diaz, referring to Brownsberger’s comment, replied, “That’s an accurate description. Everybody knows that the Senate staked out a different position originally on qualified immunity, but everyone knows that in conference committee, there are always compromises that need to be made,” the Boston Democrat said. “That’s the nature of a conference committee.”

Like Gonzalez, Chang-Diaz — the Senate’s only member of the Black and Latino Legislative Caucus — said the bill is not a complete solution but praised it as an important step. “I’m really satisfied with the total product of this bill,” she said. “It does not have everything that I want. It probably doesn’t have everything that any one of us wants. But it is a final package that strikes a lot of wins for accountability, for community voice at the table of power, for changing our vision for what public safety is and means from one of force and punishment to one of de-escalation and helping and prevention. I think there’s a tremendous amount in this bill to be proud of.”

Both she and Gonzalez said the compromise legislation met all of the demands the Black and Latino Legislative Caucus laid out this summer.

The final bill also includes language the House added through a Rep. Liz Miranda amendment allowing no-knock warrants only in cases where police have no reason to believe children or those over 65 are in a home.

Police leaders did not weigh in immediately in the wake of the bill’s release.

American Civil Liberties Union of Massachusetts Executive Director Carol Rose similarly said she “look(s) forward to studying the details of this bill. Nationwide, people have taken to the streets to end police violence, and we applaud the Beacon Hill leaders who committed early to respond to calls for reform and racial justice,” she added in a statement, expressing gratitude to lawmakers for “a bill that begins to address the challenges of this historic moment.”

Lawmakers were pressed into action by massive protests and demonstrations. In a joint statement on Monday, House Speaker Robert DeLeo and Senate President Karen Spilka said the bill is “one of the most comprehensive approaches to police reform and racial justice in the United States since the tragic murder of George Floyd.”

“Our approach strikes a balance that will provide greater protections for the rights of all residents through a strong police officer certification process via a new, independent agency, and setting clear standards for training and use of force, while providing a wider range of tools for law enforcement to provide for the safety of the public,” they said.

The conference committee is the first of six such panels privately negotiating major legislation to finish its work.

Four of the remaining five have been in business since the summer, working well past the traditional July 31 end of formal lawmaking sessions after lawmakers agreed to an extension.

Michael P. Norton and Matt Murphy contributed to this article.