The Senate on Tuesday took up the For the People Act, a sweeping bill to overhaul the country’s election system, the first step in what will probably be a long and winding legislative process. Democrats are faced with not only the all-out resistance of Republicans, but also the hesitation of their own 50th vote, Senator Joe Manchin III of West Virginia, the lone Democrat in the chamber who hasn’t signed on as a sponsor of the bill.

The elections package, which was the subject of a combative back-and-forth in the Senate Rules Committee on Tuesday, is more than a voting rights bill. You can think of it as a legislative Leviathan, designed by Democrats to attack what they see as a wide range of flaws in the country’s electoral politics in a single swoop.

But their inability to unify their own caucus around the bill — let alone to map out a path to the finish line in the face of unified Republican opposition — appears to be setting up a showdown within the party, posing a crucial test of Mr. Manchin’s so-far granite commitment to keeping the filibuster in place.

For President Biden and his party, the bill risks becoming one of the greatest disappointments of their time in power. It could be a forever what-if as Democrats strain to prevent Republicans from deepening their structural advantages up and down the country’s political system, in part by limiting who has access to the ballot.

The For the People Act contains provisions to protect voting rights, rein in big money’s role in politics, strengthen enforcement of existing election laws and limit gerrymandering. Democratic leaders have said that the bill is essential to protecting the future of democracy — particularly at a moment when Republican-led state legislatures are passing voting restrictions at a higher rate than any moment since the Jim Crow era.

At nearly 900 pages, the legislation’s enormousness may be both its greatest asset and a source of vulnerability.

“Because it’s tackling these several priorities at the same time, it has a much broader base of support from many different communities of stakeholders,” said Wendy Weiser, vice president for democracy at the Brennan Center for Justice. “I’d say that politically, the fact that it addresses more than one crisis is a strength.”

But the bill’s size leaves a lot of exposed surface area for opponents to attack, and the proceedings in the Senate on Tuesday pointed to a hard road ahead. Republican senators spent the afternoon tossing darts at the bill in the form of amendments and attacking it as a sign of Democratic overreach.

“The Democratic Party, on its own, wants to rewrite the ground rules of American politics for their benefit,” Senator Mitch McConnell, the Republican leader, said on the floor, arguing flatly that “our democracy is not in crisis.”

Mr. Manchin has expressed worries that passing an election bill along partisan lines would feed into that Republican narrative. But the bill’s supporters insist that he will ultimately have to let that go and ease his opposition to removing the filibuster, a Senate procedural tactic that has often stymied major legislation.

Norman Ornstein, an emeritus scholar at the American Enterprise Institute who has been an outspoken proponent of overhauling the filibuster, said that proceedings like Tuesday’s would “demonstrate to Joe Manchin that his desire to follow the regular order, to have a bill go through committee hearings with an open amendment process, is not going to result in a serious effort by Republicans to reach any bipartisan agreement.”

By the end of Tuesday afternoon, Republicans on the rules committee had offered up scores of amendments.

“There isn’t a whole lot of evidence that these amendments are a serious attempt to improve the bill,” Ms. Weiser said. “They’re not necessarily being offered in good faith.”

Senator Chuck Schumer of New York, the majority leader, has said that August is his probable deadline for moving the bill to a final vote — a timeline that would allow it, if passed, to take effect before the 2022 midterm elections.

But to pass election-related legislation, Democrats would need at least 60 votes, because the bill isn’t likely to qualify for budgetary reconciliation — or they would have to roll back the filibuster, which Mr. Manchin and other moderate Democratic senators remain unwilling to do.

Mr. Manchin has pushed his party’s leadership to focus on the John Lewis Voting Rights Act. That is a far narrower bill that would restore key parts of the 1965 Voting Rights Act that were struck down by the Supreme Court’s conservative majority in 2013.

But that bill would not address other aspects of what many advocates — and voters on both sides of the aisle — call a broken democratic system: campaign finance, gerrymandering and the Federal Election Commission’s ability to enforce anti-corruption and transparency laws. And even on the voting rights front, it would apply only to state-level laws passed after the John Lewis bill itself was adopted.

Fred Wertheimer, a longtime proponent of campaign-finance regulation and the founder of the advocacy group Democracy 21, said that while both bills had typically been discussed in the context of voting rights, Americans would also respond to an emphasis on the provisions in the For the People Act that take aim at money in politics and gerrymandering.

“We know that the public believes that the Washington system is rigged — and rigged against them,” Mr. Wertheimer said in an interview. “I think this will attack the problems that they perceive exist in Washington.”

“Interestingly enough, the support for this in public opinion polls is bipartisan,” he added. “The unanimous Republican opposition to this legislation exists only in the halls of Congress.”

Public opinion aside, congressional Republicans’ united resistance is what matters in the coming weeks. And that leaves Mr. Manchin firmly in the Washington spotlight.

At an event in West Virginia last week, he emphasized the need for democracy legislation and said that he was working with Representative James E. Clyburn and Senator Raphael Warnock, both Democrats, on voting legislation.

“It should not be made difficult,” Mr. Manchin said of voting. “That’s not who we are as a country. It’s not who we are as democracy. And democracy will not survive unless you have an open and fair” elections system.

Trevor Potter, a former chairman of the Federal Election Commission who now runs the Campaign Legal Center, has been promoting the bill in private meetings with lawmakers. He said that he saw no indication yet that Mr. Manchin was ready to soften his commitment to keeping the filibuster in its present form.

But the picture could change after Democrats turn to Mr. Biden’s major infrastructure proposal, an area where they have a greater chance of finding some Republican support. “I think that process, if it occurs on those bills, will be helpful on the conversations to come,” Mr. Potter said.

“Until that plays out, it’s hard to know where he’s going to end up,” he added of Mr. Manchin.