media captionNancy Pelosi: 'Sanctity of the vote is very much in peril'
US Republicans have torpedoed a Democratic bid to implement nationwide election rules, a cherished priority of President Joe Biden's party.
The bill – which sought to make it easier to vote by post – ended up deadlocked 50-50 in a party line vote.
Mr Biden said the issue was the "fight of his presidency", but some Democrats accuse him of not fighting hard enough.
Advocates say the bill would have been the most far-reaching election measure since the Voting Rights Act of 1965.
The Democrats' For the People Act passed the House of Representatives in March in a near party-line vote, with one Democrat joining all Republicans in opposing the bill.
But 60 votes are needed in the 100-member Senate to advance most legislation, and the upper chamber is evenly split 50-50 between the two parties.
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Vice-President Kamala Harris, who has been assigned by the White House to push election reform, was presiding over the chamber as the bill failed.
"The fight's not over," she said after the vote.
As well as expanding early voting, Democrats said the legislation would have ensured more transparency for certain campaign contributions and limited partisan influence over the drawing of congressional districts.
The president's party had sold the nearly 900-page proposal as an attempt to safeguard voting access for black voters.
But Republicans said the package was a federal power grab against the authority of individual US states to ensure the integrity of their own elections, designed purely to benefit Democrats politically.
The bill's legislative difficulties have reinvigorated Democratic calls for the Senate to take the radical step of eliminating a tactic known as the filibuster, allowing legislation to pass with a simple majority of 51 votes.
But at least two Democratic senators – Joe Manchin of West Virginia and Kyrsten Sinema of Arizona – have so far held firm in opposing efforts to do away with the Senate rule.
Liberals wanted Biden to try harder
There was never much doubt that the comprehensive voting rights bill was dead in the Senate.
A growing number of liberals wanted Joe Biden and his administration to try a little harder to promote the legislation, however.
Even if the president had made a stronger effort to use the "bully pulpit" to tout the benefits of the bill – and Biden did give several speeches on the subject – the chances of shifting any Republican votes was slim.
What the liberals hoped for, however, was that an intensive, ultimately unsuccessful lobbying effort would pave the way for reforming the Senate rules. Then, a simple majority could pass the voting reforms that many Democrats view as essential to blocking Republican state-level efforts to limit voting access.
Whether that was ever a possibility is debatable. But if it becomes received wisdom for those on Mr Biden's left, it could cause growing dissent in a party that will need unity to accomplish anything substantive in the days ahead.