Washington, Sep 22 (IANS): Democrats are facing hurdles as they try to pass a $3.5 trillion spending package, experts said.

The package, a sweeping social spending bill that forms the core of US President Joe Biden's domestic agenda, is a source of contention between Democrats and Republicans, the Xinhua news agency reported.

At issue is the bill's size and sources of payment. Republicans want a much smaller package, while progressive Democrats have threatened to sink any such legislation. Democrats want to raise taxes on corporations and higher earners, which Republicans believe would lead to slower economic growth.

With Congress holding a razor-thin majority, Biden will need every Democrat to be on board to get enough votes to pass the legislation.

"Biden's spending bill will pass, but at a lower level than 3.5 trillion (dollars)," Brookings Institution Senior Fellow Darrell West told Xinhua.

The final bill may end up significantly below that figure, although Democrats are likely to keep the major priorities in the bill, West anticipated.

Last week saw Biden more publicly push the negotiations forward, as lawmakers wrangled over the details of the massive bill.

Biden has trumpeted proposals to boost taxes on higher-income families and companies, which he said would pay for the legislation. He also met with Senators Joe Manchin and Kyrsten Sinema -- both moderate Democrats who harbor concerns over the bill's massive price tag, but whose votes are needed to pass the plan.

Manchin and Sinema have warned for several weeks that they are uncomfortable with the cost of the 3.5-trillion-U.S.-dollar package. That bill "will not have my vote," Manchin told CNN recently.

In a recent CNN interview, progressive Representative Alexandria Ocasio-Cortez said many progressive caucus members would not vote for Manchin's infrastructure bill "unless it is tied together" with Biden's much larger spending bill.

In an opinion piece in the New York Times, Gregory Mankiw, a Harvard economics professor and former chairman of the Council of Economic Advisers, argued that "there's good reason to doubt" the White House claims that the plan will be paid for with higher taxes on corporations and the very wealthy.

Budget experts, such as Maya MacGuineas, president of the Committee for a Responsible Federal Budget, were skeptical that the government can raise enough tax revenue from the wealthy to finance Biden's agenda.

Moreover, higher taxes "distort incentives and impede economic growth," Mankiw contended, citing this as another worry over the massive spending package Biden wants to pass.