The White House said a “potential agreement” with bipartisan negotiators in Congress was within reach on an infrastructure investment package that is expected to top $1tn, in a breakthrough that could advance a core piece of President Joe Biden’s economic agenda.

After days of talks between senior Biden administration officials and a group of centrist Democratic and Republican senators on Capitol Hill, Jen Psaki, the White House press secretary, signalled late on Wednesday that a deal could be at hand.

“White House senior staff had two productive meetings today with the bipartisan group of senators who have been negotiating about infrastructure,” Psaki said. “The group made progress towards an outline of a potential agreement, and the president has invited the group to come to the White House tomorrow to discuss this in person.”

The deal being discussed would boost funding for physical infrastructure including roads, bridges and broadband networks, but would fall far short of the more expansive $2.3tn infrastructure plan presented by Biden at the end of March.

Republicans have refused to pay for it by raising corporate taxes as proposed by the White House, so any agreement is expected to rest on redeploying unused pandemic stimulus funds and other sources of revenue to cover the cost. The final details are uncertain, however.

Even if a deal is finalised between the moderate senators and the White House, it could face some difficulty winning approval in Congress. The leadership of both parties would have to agree to it so that it could withstand defections from Republicans opposed to any new spending and from progressive Democrats who believe it is insufficient to correct the problems afflicting America’s ageing infrastructure and address climate change.

“We can’t let the infrastructure train leave the station without investing in child care and clean energy — or without making billionaires and giant corporations pay their fair share in taxes,” Elizabeth Warren, the Democratic senator from Massachusetts, wrote in a tweet on Wednesday.

If approved, the trimmed-down bipartisan infrastructure package is still expected to be followed by a more ambitious — and costly — effort by Biden to pass the rest of his economic agenda, worth at least another $3tn, solely with Democratic votes in Congress.

That would cover long-term investments in child care, education, research and development and other Democratic priorities to restructure the economy, which are expected to be paid for with higher taxes on wealthy individuals and corporations.

However, Biden’s party holds a very slim majority in the House of Representatives, and has an edge in the evenly-split Senate because Kamala Harris, vice-president, casts any tiebreaking votes. This presents a tricky political balancing act for Biden as any package would have to satisfy both wings of his party.

A bipartisan infrastructure package worth more than $1tn in spending would represent an important legislative victory for the president, who touted his ability to strike deals with Republicans during his 2020 election campaign. It would also highlight that moderate lawmakers can still drive deals on Capitol Hill if there is sufficient political will behind the effort, despite the high levels of polarisation in Congress.

Biden had initially sought to negotiate only with a group of Republican senators, but those talks fell apart earlier this month. The bipartisan lawmakers who picked up the mantle include Mitt Romney, the Utah senator, and Rob Portman, the Ohio senator. On the Democratic side Joe Manchin, the West Virginia senator and Kyrsten Sinema, the Arizona senator, led the talks.