Washington (AFP) - Joe Biden's policies always faced high hurdles with Republicans, but one Democratic senator balking at the US president's voting rights bill has signalled that the bulk of the ambitious White House agenda is under threat.

Demonized as an obstructionist within the party by some progressives, Democrat Senator Joe Manchin from ultra-conservative West Virginia also doubled down on his refusal to kill the filibuster, a blocking tactic used by the minority.

That means Biden's efforts to launch a trillion-dollar infrastructure improvement, address climate change, enshrine paycheck fairness, bolster education, reform policing and expand child and elderly care could face sabotage from within -- a crisis scenario as his party faces make-or-break midterm elections next year in a closely divided Congress.

With the Senate deadlocked 50-50 and Vice President Kamala Harris holding the tie-breaking vote, there is zero margin for defection -- a situation not lost on Manchin, who has solidified his role as the most consequential lawmaker in Congress.

Biden has said the sweeping voter rights reforms would neutralize the "unprecedented assault" on democracy in the form of laws adopted this year in 14 Republican-controlled states that restrict ballot access in the wake of the 2020 election.

Republican state legislatures have ramped up what they ostensibly call voter integrity initiatives after largely embracing ex-president Donald Trump's baseless claim that he lost because of widespread election fraud.

As many Democrats hail Biden's anti-voter-suppression measure, Manchin, 73, warned Sunday that it amounts to "partisan voting legislation (that) will destroy the already weakening binds of our democracy."

Democrats and Republicans "coming together" is the only path forward, he argued.

He also reiterated his intent to vote against any effort to weaken or eliminate the filibuster, which requires a 60-vote supermajority to advance legislation in the 100-member chamber.

Manchin's stubborn refusal to embrace the party line -- even after Republicans used their first filibuster of the year to block a bill establishing an independent commission to investigate the deadly January 6 US Capitol riot by Trump supporters -- received praise from an unlikely source.

"He's doing the right thing and it's a very important thing," Trump told Fox Business on Monday.

Biden succeeded earlier this year in ramming through a massive spending package to help the economy recover from the coronavirus pandemic. 

But two other titanic bills -- a $1.8 trillion boost for American families, and an infrastructure upgrade of between $1 trillion and $1.7 trillion -- are the subject of bitter negotiations in Congress.

Balancing act

Manchin's obstinacy, and his illusory hope of a bipartisan breakthrough, has earned harsh criticism from within the party.

"Joe Manchin is the new Mitch McConnell," House Democrat Jamaal Bowman tweeted, referring to the Senate's top Republican who has notoriously gummed up Senate work.

Bowman is one of several Democrats who see the filibuster as an antiquated, unworkable rule that has become the chief obstacle to enacting meaningful policy changes -- even those that are widely popular among voters.

The president is aware that heaping public pressure on Manchin, whose constituents are largely conservative, could backfire.

"He is literally your linchpin to having a Democratic majority, so if you push too hard, or if you force him out of your caucus one way or another, you're likely to lose that vote altogether," Casey Burgat, an assistant professor and director of the Legislative Affairs Program at George Washington University, told AFP.

Manchin is not the only Senate Democrat in the spotlight. Fellow moderate Kyrsten Sinema of Arizona has also shown an unpredictable appetite to break with party leadership. 

Sinema has spoken out against nuking the filibuster, and Burgat said she is likely quietly "pressing buttons behind closed doors" to get her views across. 

"As Sinema is showing us, they can't afford to lose one senator on any of this, which makes all of them incredibly important," he said.

Inaction on the filibuster would mean Democrats either need buy-in from 10 Republicans for Biden's agenda items, or will aim to push through major bills like infrastructure reform using reconciliation, a form of Senate trickery that allows budget-related items to pass with a simple majority.

But even going that route, Biden would need to keep people like Manchin and Sinema on board, while not losing the support of more progressive Democrats.

"It's always going to be a fine line for Democrats to appease Elizabeth Warren, Bernie Sanders and Joe Manchin all at the same time," Burgat said.