Lawmakers were set to take key votes Friday on giant twin bills at the center of Joe Biden's $3 trillion blueprint to transform America's crumbling transport network and expand the welfare safety net on the biggest day so far of his presidency.

Success on both fronts would be a huge boon to Biden, 10 months after he swept to the White House on the rallying cry of "build back better" before seeing his popularity plunge. His low approval ratings were blamed in part for a humiliating upset defeat this week in Virginia's gubernatorial election.

The House of Representatives was expected to pass a $1.2 trillion infrastructure package for Biden to sign into law after taking a procedural vote to advance a social welfare bill, worth up to $1.85 trillion, to the Senate.

"Poll after poll shows us that the components of the bipartisan infrastructure deal and the Build Back Better framework are very popular," Biden spokeswoman Karine Jean-Pierre told reporters amid a flurry of 11th-hour negotiations in Congress Thursday.

"American families want historic investments in infrastructure... in competitiveness, and addressing the climate crisis. This is something that they want to see happen."

The social spending bill -- named Build Back Better -- does not have the Senate's blessing and is likely to be downsized in the upper chamber and put through further arduous votes before the end of the month.

The US Senate passed the massive infrastructure package with the support of one-third of Republicans
The US Senate passed the massive infrastructure package with the support of one-third of Republicans AFP / Mandel Ngan

But the infrastructure deal's final approval would mark a resounding immediate victory for Biden, a former senator who touts his ability to reach across the aisle.

By funding work on roads, bridges and ports and high-speed internet, the White House says it would create thousands of high-paying jobs for people without college degrees.

The votes cap months of tense negotiations on Capitol Hill since the Senate approved the infrastructure package in August, giving it rare bipartisan support in Washington's highly-polarized political atmosphere.

Congressional Democratic leaders initially indicated they wanted the House to sync with the Senate in the summer but it was held up by wrangling over the size and scope of Build Back Better, which progressives wanted to pass at the same time.

The plan provides for $550 billion in new federal spending on transport infrastructure, but also for broadband expansion, clean water measures, electric charging stations and other measures to fight climate change.

The ambitious plan provides for $550 billion in new federal spending on transport infrastructure -- seen here is the 110 Freeway in Los Angeles
The ambitious plan provides for $550 billion in new federal spending on transport infrastructure -- seen here is the 110 Freeway in Los Angeles AFP / Patrick T. FALLON

Its top line -- the equivalent of Spain's 2020 gross domestic product -- relies on other public funds that have already been appropriated.

Needing just a simple majority, it passed the Senate by 69 votes to 30 with backing from a third of Republican Senators.

But most House Republicans are expected to withhold their support after former president Donald Trump threatened reprisals for helping to hand Biden a political win.

The bill is a key element of Biden's sweeping domestic agenda aimed at structuring a greener, more equitable economy.

Democrats in both chambers are still haggling over the Build Back Better package that includes major investments in health, education, tackling climate change and expanding social welfare programs.

Lawmakers spent Thursday making progress on multiple sticking points from the absence of a full score from the Congressional Budget Office, to the omission of prescription drug pricing curbs and pro-immigration provisions.

House Speaker Nancy Pelosi prides herself on never bringing a vote to the floor for which she has not already garnered sufficient backing, however -- suggesting she was able to twist enough arms to get consensus.

Success on Build Back Better would be something of a Pyrrhic victory, however, for lawmakers who fear it will be changed beyond recognition in the Senate by moderates including West Virginia's Joe Manchin, who continues to stiff-arm a deal.

"It's always a concern. I'm of the school that every day we delay over here and have a false start, it just empowers (Manchin) more," House Budget Committee Chairman John Yarmuth told political newspaper The Hill.

"So we need to get this over to them."