A group of House Democrats and Republicans announced Friday that they had struck a deal to establish an independent commission to investigate the Jan. 6 attack on the U.S. Capitol, a significant breakthrough after months of partisan standoff over the mandate for such a panel — and whether it should exist at all.

The proposed 10-member commission, which emulates the panel that investigated the causes and lessons of the 9/11 terrorist attacks, would be vested with subpoena authority and charged with studying the events and run-up to Jan. 6 — with a focus on why an estimated 10,000 supporters of former president Donald Trump swarmed the Capitol grounds and, more important, what factors instigated about 800 of them to break inside. Trump’s critics in both political parties view it as a means to bring further public scrutiny to his role in inspiring the violence.

“There has been a growing consensus that the January 6th attack is of a complexity and national significance that what we need [is] an independent commission to investigate,” said Rep. Bennie Thompson (D-Miss.), chairman of the House Homeland Security Committee, announcing that he had reached agreement with the panel’s top Republican, Rep. John Katko (N.Y.). “The creation of this commission is our way of taking responsibility for protecting the U.S. Capitol.”

On the heels of the commission deal, House Democrats on Friday also announced their proposal for $1.9 billion of supplemental funds to pay for security upgrades for the Capitol and other parts of the federal government.

The bill puts over a half-billion dollars toward hardening the Capitol and congressional office buildings with movable fences, door and window reinforcements, and additional security cameras and checkpoints. It also dedicates $21.5 million to stepping up security details for members facing threats, whether in Washington, their home districts or traveling between the two — and $18 million to better train and equip the U.S. Capitol Police to respond to riot situations.

But the largest part of the hefty spending bill — nearly $700 million — is simply to pay money owed to the Capitol Police, D.C. police, the National Guard and other federal agencies for costs they incurred in responding to the riot and its aftermath. It also dedicates more than $200 million to the federal courts to address threats to judges and to meet various other costs related to prosecuting those charged in connection with the insurrection.

Both the spending bill and the commission legislation face unique challenges as proponents seek to build enough bipartisan support to get the measures through both chambers of Congress. Thus far, no Republicans have voiced support for the Democrats’ spending bill. And although the commission proposal is bipartisan, and is likely to have enough backing to clear the House when it comes to a vote next week, it is unclear how many Republicans will support it.

Key GOP leaders remained dismissive of the commission proposal Friday, arguing that the panel would have to investigate more than just the Jan. 6 insurrection if it were truly to make the Capitol and those who work there safer.

“If this commission is going to come forward to tell us how to protect this facility in the future, you want to make sure that the scope, that you can look at all that what came up before and what came up after. So that’s very concerning to me,” House Minority Leader Kevin McCarthy (R-Calif.) told reporters shortly after the announcement. “[Speaker] Nancy Pelosi has played politics with this for a number of months. You’ve got to look at what the build up before and what has been going on after this.”

When reporters asked Katko whether he thought McCarthy and other leaders would have problems with the bill, the commission’s Republican co-author said he knew only that the GOP leader was reviewing the proposal and dismissed the question as inappropriate. In a printed statement, Katko said that “this is about facts, not partisan politics,” calling the measure “a fair, solid bill that will deliver answers on the federal response and preparedness to ensure nothing like this happens ever again.”

Earlier commission negotiations between House Speaker Nancy Pelosi (D-Calif.) and congressional GOP leaders broke down when Republicans demanded that the panel look into far-left radicalism as well as the right-wing and white-nationalist groups that promoted and populated the rally Trump held outside the White House on Jan. 6 and the subsequent march on the Capitol. They also objected to Democrats’ having sought to grant themselves more power to appoint panel members than the minority Republicans — a point Pelosi had said she would be open to change.

In the bipartisan deal announced Friday, five members would be appointed by the Democratic leaders of the House and Senate, while the other five would appointed by their Republican counterparts. In a departure from Pelosi’s earlier proposal, President Biden would not have a say in appointing any commissioners.

The commission would have the power to subpoena witnesses, but not without an agreement between the Democratic-appointed chair and the Republican-appointed vice chair, or a majority vote of the panel. Current government officials, including those holding elective office, would not be allowed to serve on the panel, to maintain its independence.

The commission would be tasked with producing a final report detailing its findings, as well as any recommendations for preventing similar attacks in the future, by the end of this year, giving it about six months to complete its workif Congress approves the commission in short. By comparison, the 9/11 Commission took 20 months to publish its findings.

In his announcement, Thompson predicted that legislation would be on the House floor as soon as next week. While it is almost certain to gain majority support from the Democratic-led House, Republicans will have to decide whether to side with moderates like Katko or with GOP leaders who have resisted efforts to hold Trump accountable for the violence. Many Republicans fear that supporting the commission risks alienating the former president’s followers, a base of political support they consider vital to reclaiming majorities in the House and Senate.

Just this week, House Republicans ousted Rep. Liz Cheney (R-Wyo.) from the No. 3 spot in their leadership over her public rebukes of Trump for promoting the “big lie” that President Biden stole the election from him. On Friday, just moments before Thompson and Katko announced their breakthrough on the commission, the GOP conference voted to replace Cheney with Rep. Elise Stefanik (R-N.Y.), who has a more moderate voting record but has been vocal in her support for Trump and his spurious claims that widespread electoral fraud marred the election in November.

Even in his post-presidency, Trump has continued to feed his supporters misinformation suggesting that Biden’s win is illegitimate.

Cheney told reporters Friday that she thought “it’s great the speaker has announced the January 6 commission, and I hope we’ll be able to really have the kind of investigation we need about what happened.” She also told ABC News that McCarthy should “absolutely testify” in the course of any commission’s investigation.

During a phone call on Jan. 6, later brought to light by Rep. Jaime Herrera Beutler (R-Wash.) during Trump’s second impeachment trial, the president rebuffed McCarthy’s pleas to call his supporters back from the Capitol during the riot. “Well, Kevin, I guess these people are more upset about the election than you are,” Trump told McCarthy, according to what Herrera Beutler said the GOP leader later told her.

Cheney told ABC News that McCarthy “very clearly, and said publicly, that he’s got information about the president’s state of mind that day.”

Cheney, Katko and Herrera Beutler are among 10 House Republicans who voted with Democrats in January to impeach Trump on a charge of inciting the insurrection against Congress as it worked to tally the electoral college results. He was later acquitted by the GOP-controlled Senate, though seven Republicans crossed party lines and voted in favor of convicting him.