Barack Obama says wealthy Americans – including himself – can afford tax increases to help fund Joe Biden’s ambitious spending plan.

“I think they can afford it,” Obama told ABC’s Good Morning America. “We can afford it. I put myself in this category now.”

Obama is reckoned to have a personal fortune of around $70m. Since leaving the White House in 2017 he has released a bestselling memoir as part of a $65m book deal and with his wife, Michelle Obama, signed a Netflix deal thought to be worth more than $100m. He spoke to ABC to mark the start of construction at his presidential library, in Chicago.

Republicans and business groups oppose corporate and personal tax increases in the Biden spending package, which is priced at $3.5tn and aims to improve healthcare, childcare, the fight against climate change and other Democratic priorities.

“You’re talking about us stepping up and spending money on providing childcare tax credits,” Obama said. “Making those permanent to help families, who for a long time, have needed help.

“You’re looking at making our infrastructure function more efficiently … you’re talking about rebuilding a lot of buildings, roads, bridges, ports so that they are fortified against climate change. And also, that we start investing in the kinds of energy efficiency that’s going to be required to battle climate change.”

The House speaker, Nancy Pelosi, has indicated the price tag of the spending plan will come down, via talks between moderates and progressives also aimed at passing a bipartisan infrastructure bill. Most observers say failure to pass the Build Back Better Act will be disastrous for Biden and his party before midterm elections next year.

The spending plan must pass via reconciliation, which allows budgetary legislation to pass on bare majorities, negating the 60-vote filibuster in the evenly split Senate. Pelosi, the Senate majority leader, Chuck Schumer, and the Biden White House have little room for error.

Amid coverage of such a high-stakes political moment, the CNN commentator Chris Cilizza said Biden, a senator since 1973 and twice a presidential candidate before winning the White House in 2020, was now in “the most important week of his life”.

Biden was vice-president to Obama, who remains broadly popular.

In an intervention perhaps designed to appeal to Democratic loyalties, he told ABC Biden was “asking the wealthiest of Americans, who have benefited incredibly over the last several decades – and even in the midst of a pandemic, saw their wealth and assets rise enormously – to pay a few percentage points more in taxes in order to make sure that we have a economy that’s fair for everybody.

“I think anybody who pretends that it’s a hardship for billionaires to pay a little bit more in taxes so that a single mom gets childcare support or so that we can make sure that our communities aren’t inundated by wildfires and floods and that we’re doing something about climate change for the next generation – you know, that’s an argument that is unsustainable.”

Nonetheless, it is an argument Republicans and corporate groups are prepared to mount. In Congress, Republicans oppose attempts to simultaneously fund the government past Friday and raise the debt ceiling, thereby avoiding a US default.

Democrats have pointed to their votes to raise the ceiling under Donald Trump, and to Republicans’ contribution to the national debt via 2017 tax cuts which disproportionately benefited the wealthy. On Monday House Democrats wrote to Mitch McConnell, the Senate minority leader, urging him to “avert a manufactured crisis”.

Later, the Senate voted on a measure to fund the government and raise the debt ceiling. Republicans blocked it.