WASHINGTON (AP) — The Democrats’ expansive elections and voting bill headed for all but certain rejection late Tuesday in a key Senate test vote, providing a dramatic example of Republicans’ use of the filibuster to block legislation and forcing hard questions for Democrats over next steps.

The far-reaching proposal, at nearly 900 pages, is viewed by backers as the civil rights issue of the era, legislation that is suddenly of the highest priority after the 2020 election as states impose restrictive new laws that could make it more difficult to vote. In the evenly split Senate, Republicans are united in opposition, seeing the bill as federal overreach and denying Democrats the 60 votes that would be needed to overcome the filibuster and begin debate.

Hours before the vote, Sen. Joe Manchin, a moderate West Virginia Democrat who had been the party's lone holdout, unified Democrats in support of advancing the bill after others in the party agreed to consider changes he had sought.

“These reasonable changes have moved the bill forward and to a place worthy of debate on the Senate floor," Manchin said in a statement. “Unfortunately, my Republican colleagues refused to allow debate of this legislation despite the reasonable changes made to focus the bill on the core issues facing our democracy.”

Republicans showed no sign of yielding.

Republican leader Mitch McConnell called the bill a “partisan power grab” and pledged to help fulfill the Senate's “founding purpose” by stopping it. He also cautioned Democrats against changing the filibuster, calling voting rights “the worst possible place” to “trash the Senate’s rules to ram something through.”

Months in the making, Tuesday’s showdown over the For the People Act, as it is called, is hardly the end of the road. President Joe Biden has vowed what the White House calls the “fight of his presidency” over ensuring Americans' access to voting. At stake are not only election rules that make it easier to vote but also Democrats' ability to confront the limits of bipartisanship and decide whether or not the filibuster rules should change.

The Democratic Party that holds the White House and narrowly controls Congress has been preparing for this moment for months, even as its lawmakers faced their own internal divisions over the sprawling bill. The legislation would remove hurdles to voting erected in the name of election security, curtail the influence of big money in politics and reduce partisan influence over the drawing of congressional districts.

As recently as last week, Manchin said he couldn't support the bill without changes he wanted as a way to draw Republican support.

His position has since evolved. He proposed his own changes last week, adding provisions for a national voter ID requirement, which is anathema to many Democrats, and dropping a proposed public financing of campaigns.

The proposed changes sought by Manchin were welcomed by Biden's administration as a “step forward" and earned a nod of approval from one of the party's key voting rights advocates, former Georgia governor's race candidate Stacey Abrams.

They did little, however, to garner the bipartisan support Manchin was hoping for. Senate Republicans said they would likely reject any legislation that expands the federal government's role in elections.

“I keep thinking there’s a few who want to,” Jeff Merkley, D-Ore., who authored the legislation in the Senate, said during a conference call Monday night with the group Our Revolution. “But when McConnell lowered the boom,” he continued, “we couldn’t get a single Republican to join us.”

The rock-solid opposition from the GOP senators brings to a head questions over the filibuster, the decades-old Senate rule that requires 60 votes for most bills to advance.

Sen. Raphael Warnock, a Georgia Democrat and senior pastor at the Atlanta church Martin Luther King Jr. once led, called minority Republicans’ intention of blocking debate a “dereliction” of duty.

“What could be more hypocritical and cynical than invoking minority rights in the Senate as a pretext for preventing debate about how to preserve minority rights in the society,” Warnock, who is Black, said during a floor speech Tuesday.

While some Democrats want to change the Senate's rules to push the elections bill and other priorities past the filibuster, Manchin and others including Sen. Kyrsten Sinema of Arizona are opposed. Biden, too, has said in the past that he wants to leave the filibuster intact.

“The filibuster compels moderation and helps protect the country from wild swings," Sinema wrote Monday in an opinion piece for The Washington Post. She welcomed a full debate “so senators and our constituents can hear and fully consider the concerns and consequences.”

Pressure to change the rule is mounting, though. For now, White House press secretary Jen Psaki said the administration’s hope is that the chamber’s 50 Democrats are aligned and that an unsuccessful vote will prompt the search for a new path.

The White House didn’t give its full support to the Manchin alternative. But Psaki said the president “is appreciative of the efforts by Sen. Manchin and others to continue to make progress on voting rights, which he feels is a huge priority.”

The bill is taking on fresh urgency as former President Donald Trump continues to challenge the outcome of the 2020 election and is urging the new laws in Republican-led states.

State officials who certified the results of the 2020 election have dismissed Trump’s claims of voter fraud, and judges across the country have dismissed multiple lawsuits filed by Trump and his allies. Trump’s own attorney general said there was no evidence of widespread fraud that would change the outcome.

The changes being enacted in many of the Republican states are decried by voting rights advocates who argue the restrictions will make it more difficult for people to cast ballots, particularly minority residents who tend to support Democrats.

As the Senate action churns, more changes could be coming to the bill.

Democrats want to protect against intimidation at the polls in the aftermath of the 2020 election. They propose enhancing penalties for those who would threaten or intimidate election workers and creating a “buffer zone” between election workers and poll watchers, among other possible changes.

They also want to limit the ability of state officials to remove a local election official without cause. Georgia Republicans passed a state law earlier this year that gives the GOP-dominated Legislature greater influence over a state board that regulates elections and empowers it to remove local election officials deemed to be underperforming.

“There’s a rot, a rot, a the center of the modern Republican Party,” said Senate Majority Leader Chuck Schumer, hours ahead of the expected vote. “Should the United States Senate even debate how to protect the voting rights of our citizens? There’s only one correct answer. We’ll see if our Republican colleagues choose it this afternoon.”