Biden struck a confident note on his $US1 trillion ($A1.3 trillion) bipartisan infrastructure deal and a separate, social spending plan expected to cost under $US2 trillion during a CNN town hall event in Baltimore on Thursday.

The legislation is at the heart of his domestic agenda.

When asked on CNN whether Democrats were close to a deal, Biden said: "I think so." Later, he said: "If we can't eventually unite this country, we're in deep trouble ... I do think I'll get a deal."

White House spokeswoman Karine Jean-Pierre told reporters that Biden would insist the legislation be paid for and not add to the deficit, which will be difficult to do without the tax increases he pushed as part of his presidential campaign last year.

On Thursday, Biden said corporate tax rates were unlikely to be hiked in a spending bill. Instead, he said a separate minimum corporate tax proposal could fund the social programs.

The social spending plan remains a subject of pitched debate on Capitol Hill and in the White House as negotiators look for the sweet spot between progressives wanting an array of new programs and moderates worried about the cost.

As Biden seeks a final agreement in coming days, questions have emerged about whether some of his most oft-cited promises, such as raising taxes on corporations and wealthy Americans might have to be dropped to ensure passage of the spending bill.

Negotiations now centred on four or five issues, Biden said, declining to give further details. He later said a clean energy performance plan had not been dropped by the bill, adding that Senator Kyrsten Sinema was "very supportive" of his environmental agenda.

Sinema and Senator Joe Manchin, both moderate Democrats, have been pushing for a smaller package and have opposed some elements of the bill.

Australian Associated Press