Washington (AFP) - Joe Biden faces the most important test of his presidency this week as Democrats in the US Congress launch a highwire bid to implement his sweeping economic agenda while keeping the government's lights on.

The House and Senate are moving toward votes on legislation dealing with infrastructure and social programs worth almost $5 trillion while also averting a government shutdown on Friday and a looming debt default.

Failure on any front would be catastrophic for a president looking to cement his legacy, while Democrats would see their chances diminished for hanging onto the House of Representatives and Senate in next year's midterm elections.

"You know me: I'm born optimistic. I think things are going to go well. I think we're going to get it done," an upbeat Biden told reporters at the White House.

At stake is the fate of Biden's $3.5 trillion Build Back Better social welfare package that only Democrats support, and a $1.2 trillion infrastructure bill to improve transport networks and broadband coverage that has already passed the Senate with cross-party support.

Democratic House Speaker Nancy Pelosi told colleagues that there would be a vote on infrastructure on Thursday -- the same day Congress needs to strike a funding deal to avoid a government shutdown.

'Intensity'

Progressive Democrats in the House have repeatedly threatened to tank infrastructure without a significant commitment from their moderate colleagues on Build Back Better.

But moderates nervous about freewheeling spending have said they may not vote for the bigger bill without infrastructure crossing the line first -- and have talked about scaling back the $3.5 trillion ticket price in any case.

"The next few days will be a time of intensity," Pelosi wrote to House Democrats, who have a three-vote margin in the House, plus the anticipated support of at least 10 Republicans for infrastructure.

Biden says the two bills would save the average family $15,000 a year through expanded care and education programs and reduced drug pricing, while transforming public transport, highways, bridges and broadband internet access would make life easier and greener.

The president told reporters Monday that agreement on Build Back Better may not be completed before the infrastructure vote and shutdown deadline, although he added: "I hope it's by the end of the week."

'Week from hell'

Adding to the drama, lawmakers have until midnight going into Friday to green-light a package to fund the government ahead of federal agencies running out of money.

Failure would shut down the government, which typically leads to hundreds of thousands of workers being sent home as public services are closed.

The House passed a "continuing resolution" last week to keep the government open until December 3 -- but the evenly-divided 100-member Senate rejected the package on Monday, with Democrats failing to secure the 60 votes necessary to begin debate.

Republicans are opposed to a 14-month debt ceiling hike that was included in the wording following warnings that the Treasury Department will be unable to obtain new loans sometime in October.

They say Biden's spending plans are out of control, and that the Democrats -- who control both chambers of Congress -- should avoid the disastrous looming credit default by lifting the borrowing cap on their own.

In reality, the debt limit suspension would cover spending backed by Republicans under Donald Trump, as well as other spending already authorized by the government.

Biden, who discussed the bills via phone earlier Monday with Pelosi and Senate majority leader Chuck Schumer, slammed what he called Republican "actions that show a lack of concern about our economic recovery," according to a White House statement.

The upper chamber is now likely to send a "clean" continuing resolution back to the House to pass without the debt-limit provision, averting the shutdown for two months.

That would leave the debt-limit problem unresolved, however -- a worrying outcome that will make the markets increasingly jumpy as the mid-to-late October deadline draws closer.

"You want me to be honest? It's going to be a week from hell," Rep. Debbie Dingell told CNN on Sunday.

"But failure is not an option. There is too much at stake."