Democrats’ hopes of finally pushing through voting rights legislation after months of Republican opposition appeared to be fatally wounded Thursday after Sen. Kyrsten Sinema (D-Ariz.) announced she would not support changing Senate rules that have long allowed a minority of senators to block legislation.

Sinema’s position, outlined in a midday floor speech, echoed her previous public statements where she defended the filibuster, the Senate’s 60-vote supermajority rule, as a tool to facilitate bipartisan cooperation and guard against wild swings in federal policy.

But the circumstances in which she reiterated it — as Senate Democratic leaders prepared to launch a decisive floor debate and less than an hour before President Biden was scheduled to arrive on Capitol Hill to deliver a final, forceful appeal for action — put an exclamation point on her party’s long and fruitless effort to counter restrictive Republican-passed state voting laws.

“While I continue to support these bills, I will not support separate actions that worsen the underlying disease of division infecting our country,” Sinema said.

She later added: “We must address the disease itself, the disease of division, to protect our democracy, and it cannot be achieved by one party alone. It cannot be achieved solely by the federal government. The response requires something greater and, yes, more difficult than what the Senate is discussing today.”

Biden, a former six-term senator, is delivering his closed-door pitch two days after he made his most pointed public case yet for the modification or elimination of the filibuster to pass voting rights bills. He chose to come and make a final push even as Sinema and Sen. Joe Manchin III (W.Va.) had shown no indication that they are prepared to move off their long-standing public opposition to weakening minority rights in the Senate.

Still, Democrats are prepared to move forward with a floor confrontation in the coming days that could bring their year-long quest to counter Republican-passed state voting restrictions to a futile and frustrating end.

Senate Majority Leader Charles E. Schumer (D-N.Y.), who called the struggle to persuade Manchin and Sinema “an uphill fight” in interviews this week, laid out a plan Wednesday to bring two key voting rights bills to the Senate floor in the coming days, setting up a final showdown over the rules early next week.

Speaking on the Senate floor Thursday morning, Schumer said Democrats had no choice but to move forward unilaterally after trying to secure Republican cooperation on voting rights for months.

“Every step of the way, we’ve been met with near total resistance,” he said. “Members of this chamber were elected to debate and to vote — particularly on an issue as vital to the beating heart of our democracy as this. I have said for months that just because Republicans have refused to work with us to protect voting rights does not mean Democrats would stop working to move forward on our own. The matter is simply too important.”

The House attached the voting legislation to a bill previously passed by the Senate in a Thursday-morning vote and sent it back across the Capitol, a maneuver that will allow Senate Democrats to skip procedural hurdles and quickly start debate on the Senate floor.

“Nothing less than our democracy is at stake,” House Speaker Nancy Pelosi said in a letter to fellow House Democrats on Wednesday. With Thursday’s vote, she said, “the Democratic House will make clear: we stand with the President — and with the people — to #FightForVotingRights.”

As the House debated Thursday morning, Senate Minority Leader Mitch McConnell (R-Ky.) accused Democrats of focusing on the divisive and distracting issue of voting rights and Senate rules instead of more pressing challenges to the nation’s economy and security.

“Nobody in this country is buying the fake hysteria that democracy will die unless Democrats get total control,” he said. “There’s a path forward for my Democratic colleagues to respond to the country they have so badly disappointed, but it isn’t to try to break the Senate and rewrite election laws, it’s to actually start tackling the issues that American families need tackled.”

After Sinema spoke Wednesday, McConnell said he “couldn’t agree more with her decision,” while other Republicans marveled that Democrats brought the issue to a head knowing Sinema’s long-standing views on the filibuster.

“She’s just expressing a frustration that they don’t appear to be listening to her,” said Senate Minority Whip John Thune (R-S.D.). “She’s been clear. She’s been public from the beginning about this, and they seem to continue to think that somehow they’re going to switch her, and the more they do that, I think the more dug in she’s gotten.”

More than a half-dozen Republicans, including top leaders McConnell and Thune, watched Sinema’s speech inside the Senate chamber, while only two Democrats opted to attend in person.

Delivering the speech wearing purple, a symbol of Washington bipartisanship, Sinema said she wished there had been “a more serious effort on the part of Democratic Party leaders to sit down with the other party and genuinely discuss how to reforge common ground” on voting rights issues. But she also said Republicans “have a duty to meet their shared responsibility to protect access to voting and the integrity of our electoral process.”

“Eliminating the 60-vote threshold on a party line with the thinnest of possible majorities to pass these bills that I support will not guarantee that we prevent demagogues from winning office. Indeed, some who undermine the principles of democracy have already been elected,” she said. “Rather, eliminating the 60-vote threshold will simply guarantee that we lose a critical tool that we need to safeguard our democracy from threats in the years to come.”

The confrontation has been nearly a year in the making, initially spurred by new voting laws passed by Republican state legislatures starting early last year that sought to scale back voting by mail, early voting, ballot drop boxes and other provisions that have made voting more convenient and accessible but which became a central focus of former president Donald Trump’s false claims that the 2020 presidential election was stolen.

Civil rights groups with close ties to Democratic Party leaders have been especially forceful in calling for action, warning that the GOP state laws threaten to depress minority turnout and swing future elections.

Republican leaders in Washington have aggressively countered the push, arguing that the state laws are justified to restore faltering public confidence in elections and that the federal government has no role in dictating how states run their elections.

In fact, the Constitution gives Congress wide latitude to regulate the conduct of federal elections, and the Senate has tried four times over the past year to pass voting rights bills. Each time, with one exception, Republicans banded together to block their advancement. (Sen. Lisa Murkowski of Alaska voted to advance one bill, the John Lewis Voting Rights Advancement Act, in November.)

The failed efforts have spurred rising interest among Democrats in changing the Senate rules to prevent future GOP blockades, and several senators who defended the filibuster under previous Republican Senate majorities have publicly said they favor creating an exception to pass a voting rights bill. Some have argued for even more thoroughgoing reforms that would change the nature of the filibuster entirely by forcing the objecting minority actually hold the floor and speak rather than simply register a silent blockade.

Biden, in a voting rights speech delivered Tuesday in Atlanta, referred to his 36 years of Senate service, describing himself as an “institutionalist” who has seen the institution wither under the stresses of political polarization, leading to an increasingly routine deployment of filibusters and to an increasingly sclerotic legislative process.

“Sadly, the United States Senate — designed to be the world’s greatest deliberative body — has been rendered a shell of its former self,” he said, calling the filibuster “weaponized and abused.”

Biden received fresh backing Wednesday from former president Barack Obama, who has previously questioned the filibuster but made his most forceful call for action in a USA Today op-ed published Wednesday night that called on the Senate to change its rules and pass the voting rights legislation over GOP objections.

“In recent years, the filibuster has become a routine way for the Senate minority to block important progress on issues supported by the majority of voters. But we can’t allow it to be used to block efforts to protect our democracy,” Obama wrote. “That’s why I fully support President Joe Biden’s call to modify Senate rules as necessary to make sure pending voting rights legislation gets called for a vote. And every American who cares about the survival of our most cherished institutions should support the President’s call as well.”

While Manchin and Sinema have been closely involved in discussions with their colleagues about the Senate rules, they have shown little appetite for breaching the 60-vote rule, which has been eroded over the years for executive nominations, budget legislation and other limited circumstances.

Manchin this week told reporters that he was not willing to change the rules without GOP buy-in, and he again defended the importance of the supermajority threshold, saying senators “need to be very cautious [about] what we do.”

“They’re coming down to crunchtime and understand the position they’re in and what they’re doing,” he said of his fellow Democrats. “But I think I’ve been very clear where I am, you know, so I hope they respect that, too.”

Sinema has made clear she supports federal legislation on voting rights but has consistently expressed reservations about changing Senate rules to pass it — a position that only solidified after she led a group that negotiated a bipartisan infrastructure bill this summer.

Last month, her office issued a statement confirming that she “continues to support the Senate’s 60-vote threshold, to protect the country from repeated radical reversals in federal policy which would cement uncertainty, deepen divisions, and further erode Americans’ confidence in our government.” She has warned publicly about the prospect that Republicans could pass severe national voting restrictions should they win the congressional majorities that Democrats enjoy now.

As the debate among Democrats has intensified in recent weeks, the tensions between Democrats and Republicans have risen as well. GOP senators have accused Democrats of hypocrisy for reneging on their previous defenses of the filibuster, and they have warned that any party-line changes to the rules will backfire — prompting hardball floor tactics and an eventual turning of the tables once Republicans regain power.

In his speech Tuesday, Biden compared the GOP opponents of the Democratic voting rights bills to those who stood against civil rights for African Americans in earlier, more violent moments in U.S. history.

“I ask every elected official in America: How do you want to be remembered?” he said. “Do you want to be the side of Doctor [Martin Luther] King or George Wallace? Do you want to be on the side of John Lewis or Bull Connor? Do you want to be on the side of Abraham Lincoln or Jefferson Davis?”