On Tuesday, 19 Republican senators, including minority leader Mitch McConnell, joined with Democrats to pass Joe Biden’s $550bn infrastructure bill. In a polarised age, this act of bipartisan politics seems miraculous. To vote for the bill, Senate Republicans had to go against the wishes of Donald Trump, who had warned against handing Mr Biden a victory before midterm polls in 2022. They also U-turned on a core Republican principle: that private investment is superior to government intervention.

Yet the Republicans’ vote was rooted in self-interest. Only four will face the voters next year and the spending was popular, even with Republicans. Crucially Mr McConnell had protected the filibuster. Unless Republicans relented, Mr Biden might have done away with legislative tool that preserves the Senate’s 60-vote threshold for legislative success. Instead Mr Biden thanked his opponents for their courage in backing his proposal. This moment represents a test of Biden’s faith that Congress, and democracy, can still work and get things done.

In many ways this looks like a defining battle for the heart and soul of the Democratic party. The infrastructure bill now goes to the House of Representatives, which has a Democratic majority and a bigger progressive bloc. The House Democratic leadership has said it will only move after the Senate passes a $3.5tn ​​spending bill to reduce poverty, improve elderly and childcare as well as protect the environment. The biggest expansion of the US’s social safety net since the Great Society of the 1960s is needed to help flatten the inequalities wrought by decades of pro-market policies. The same can be said for rolling back the tax cuts for corporations and wealthy households that were Mr Trump’s signature legislative achievement.

It is important to note that leftwing Democrats have had to trim their demand for a $6tn package. But some on the right of the party appear more in tune with Republican arguments that characterise the $3.5tn bill as “reckless”. After agreeing to vote for the bill’s framework, West Virginia’s Joe Manchin said he had “grave concerns” about such a price tag. Kyrsten Sinema of Arizona last month made it clear she could not support a bill that size. They are not the only ones: in the House moderate Democrats would rather take an easy win and dump any attempt to enact big, bold social change.

The criticism the US cannot afford the spending is wrong. The economist Stephanie Kelton wrote that Mr Trump’s tax cuts added $1.9tn to the country’s fiscal deficit with little effect on the country’s ability to spend. The other concern is inflation. Prof Kelton noted many experts thought “Congress could enact both the bipartisan infrastructure bill and the proposed $3.5tn reconciliation bill without exacerbating inflation”.

Perhaps the greatest obstacle to Mr Biden’s ambition is not the politicians, but the ideological orientation of the Congressional Budget Office (CBO), which scores the spending and revenues. Under reconciliation rules, measures cannot add to the deficit after a decade. In a sign of what lies ahead, Mr Biden’s treasury team has already claimed that tax enforcement will raise more cash than the CBO projects. The president knows that the New Deal and Great Society programmes passed into law without a CBO score. Mr Biden would like to change America on a such a scale. But transformations like that cannot be bought. They must be fought for.