Bernie Sanders, the leftwing firebrand who has drawn the fight against poverty and inequality into the mainstream of American politics, issued a call to arms on Sunday for fellow progressives to stand firm in the intensifying battle over the future of Joe Biden’s economic and social policy agenda.

With the Democratic party locked in a bitter struggle over two massive legislative bills that could make or break the Biden presidency, Sanders said the outcome of the next few weeks would be critical not just for the future of American working families but also for the country’s political future.

“This is a test of whether American democracy can work,” he said in a fiery interview with ABC News’s This Week. “I hope and expect that the Democratic party and the president – I know he will – will stand firm.”

With the White House and congressional leaders scrambling to meet a new 31 October deadline for passing the legislation, two flanks of the Democratic party find themselves at loggerheads. They are lined up over the fate of Biden’s two key legislative measures: the $1tn infrastructure bill and the $3.5tn social and environmental plan.

In one corner of the ring, key moderates including Joe Manchin and Kyrsten Sinema in the US Senate and Josh Gottheimer in the House complain that the $3.5tn so-called reconciliation bill is too large and argue for the smaller infrastructure bill to go ahead on its own.

In the other corner, leading progressives – Sanders together with the chair of the progressive caucus Pramila Jayapal and others – demand that the two bills should be voted on together in order to give the reconciliation plan the best chance of passing.

On Sunday Sanders was bullish about the chances of his side prevailing. “We have the American people very strongly on our side, we have the president of the United States on our side, we have 96% of the Democratic caucus of the House on our side, and we’ve got all but two senators on our side. We’re going to win this thing,” he told This Week.

The current impasse over Biden’s legislative program brings the independent senator from Vermont back into the political limelight. In his first presidential run against Hillary Clinton in 2016 he took the country by storm, acquiring an army of young supporters chanting “Feel the Bern” and attracted to his radical platform of a $15-an-hour minimum wage and other policies to reduce income inequality.

In 2020 Sanders began as the Democratic frontrunner but was later outgunned by Biden. Having conceded defeat, Sanders went on to endorse Biden and campaign for him in his bid to unseat Donald Trump.

The alliance between the two top Democrats appears to be holding through the current turmoil. Both men have described the reconciliation measure as the “most consequential piece of legislation since the Great Depression”.

Sanders spoke passionately about some of the plan’s central proposals, notably moves to tackle the “existential threat” of the climate crisis. He said: “Scientists are telling us that if we do not act boldly in terms of cutting carbon emissions then the planet we are leaving our kids and grandchildren will be increasingly uninhabitable.”

He also vividly highlighted the proposal to widen Medicare for older Americans to include dental treatment. “Elderly people in this country cannot chew their food because they don’t have teeth in their mouth,” he said.

Sanders argued that much of what is contained in the social plan is popular among most American voters.

“What bothers me about this whole thing is that poll after poll shows what we are doing is exactly what the American people want. It’s not what the big money interests want, it’s not what the lobbyists want, it’s what the American people want – and we’ve got to do it,” he said.

Sanders’ claim is borne out by opinion polls which show more than half of US voters back the massive social overhaul package which, if passed in its present form, would stand as the largest expansion of social welfare in the US since the 1960s.

The fierce struggle now gripping the Democratic party could be seen as the climax of Sanders’ decades-long effort to focus public attention on America’s exceptionally weak social safety net. If the White House succeeds in passing the social overhaul package with some of its most important elements intact, then Sanders could claim vindication for a position that for many years confined him to the political margins.

To ensure that happens Sanders said he is willing to do his part to reach a compromise. Having initially pitched his proposal at $6tn – a sum which he still insists was even too little to address the climate crisis – he now concedes “there will have to be some give and take”.

But being Sanders, a self-described democratic socialist, he continues to express his politics in the most expansive terms.

He told This Week: “We are not just taking on senators Manchin and Sinema, we are taking on the entire ruling class of this country. Right now the drug companies, the health insurance companies, the fossil fuel industries are spending hundreds of millions of dollars to prevent us doing what the American people want.”