President Joe Biden went on the offensive Monday with a speech attacking Republicans over a looming threat of US debt default, while pressuring his Democratic Party to enact his stalled multi-trillion-dollar domestic spending agenda.

Back from a rare weekend relaxing at home in Delaware, Biden plunged into the most consequential period of his presidency so far.

On one side, he faces Republican determination to cripple his momentum and recapture control of Congress in next year's midterm legislative elections. On the other, Biden is struggling with infighting between Democrats over his infrastructure and social spending bills.

With the speech calling out Republicans and a trip to Michigan on Tuesday to promote his domestic spending plans, the 78-year-old political veteran hopes to regain the initiative.

While Biden's legacy may ultimately depend on the $1.2 trillion infrastructure package and a potentially $2 trillion or more social spending package, the entire US and global economies face the imminent threat of a possible debt default.

Biden on Monday called Republican opponents "reckless and dangerous" for refusing to join Democrats in raising the debt limit.

Republican obstruction could push "our economy over a cliff," Biden said in a White House speech, warning he cannot "guarantee" that a resolution will be found in time.

"If I could, I would," he said.

Treasury Secretary Janet Yellen warns that from October 18, the United States will not have the funds to meet its obligations to creditors if Congress does not relax the legal debt ceiling.

Congress has done this dozens of times over the decades since setting borrowing limits, and the votes are usually bipartisan and drama-free.

This year, reflecting the extraordinary acrimony, Republicans are refusing to vote for lifting the ceiling and indicate they will even block Democrats from passing a simple vote by themselves along party lines.

Instead, Senate Republicans are attempting to force Democrats to use a complex maneuver called reconciliation to take sole responsibility for the debt hike. Democrats so far are refusing, accusing the Republicans of taking the nation's financial standing hostage.

The standoff means that Democrats, who control the Senate by only one vote, find themselves bogged down in trying to manage the debt crisis while also trying to overcome internal differences over Biden's spending packages.

On Monday, Senate Majority Leader Chuck Schumer said a debt ceiling lift should be voted through "by the end of the week, period."

"We do not have the luxury of waiting until October 18th, as it is our responsibility to re-assure the world that the United States meets our obligations in a timely fashion," he said.

How much is enough? 

Meanwhile, Biden is needing every bit of his experience from nearly four decades in the Senate and eight years as vice president under Barack Obama to try and come up with a formula that will unite the left and more conservative wings of his party.

His trip Tuesday to a trade union training facility in Howell, Michigan, will seek to highlight the White House's argument that the big spending plans are popular with voters and that Democrats would be committing colossal self-harm if their squabbling results in the entire legislative agenda collapsing.

On Thursday, Biden is also set to fly to Chicago to talk about the Covid pandemic -- underlining his argument that the country needs effective government at a time of national crisis.

Moderates in the House and most crucially in the ultra-tight Senate are refusing to go along with the progressive wing's hoped-for $3.5 trillion price tag on social spending. Progressives are rejecting a counter-offer of $1.5 trillion.

Biden is now pushing for something in the $2 trillion range.

However both camps are playing hardball, with progressives refusing to back even the $1 trillion infrastructure component unless their bigger social spending goals are first guaranteed.

On Sunday, Schumer said the goal was "to get both bills done in the next month," adding yet another deadline to a tense autumn season for Biden's team.