In the last few weeks, centrist Democrats, including former US president Barack Obama, have expressed concern about Democratic primary candidates moving too far to the left, as Senators Bernie Sanders and Elizabeth Warren, the top progressive candidates, continue to command a major share of grassroots support. Centrists worry that progressives will not be able to mobilise the wider Democratic base or secure enough swing voters to defeat Republican President Donald Trump in the general election next year.

For these centrists, the election’s main goal is to defeat Trump. Progressives, on the other hand, believe that it is not enough to defeat Trump but that they must also reform the political system of which he is a symptom. In their view, if the system, which has been corroded by moneyed interests, is not fixed, it will lead to greater economic inequality and environmental collapse. Centre-left candidates such as Joe Biden and Michael Bloomberg have significant financial backing, but in the last three months Sanders and Warren raised more money than the other Democrat candidates, mostly through small donations made online. This suggests a level of grassroots support that their rivals cannot match, and an enthusiasm in the electorate for greater political change than is offered by those closer to the political centre.

The Democrats have now had three years to reflect on their previous choice of a centrist candidate, Hillary Clinton, who was defeated by Trump in 2016. Many reflected at the time that by playing it safe, they lost the presidency. Among these was Trump’s own pollster, Tony Fabrizio, who commented after that election that if Sanders had been the Democratic nominee, he would have defeated Trump.

Sanders is widely seen as the standard-bearer of the progressive movement that has gathered pace within the Democratic Party since his campaign for nomination in 2016. Sanders and his fellow progressive, Elizabeth Warren, are the only two candidates to tie with the presumptive frontrunner, Joe Biden, in the polls.

While Warren has received significant media attention, early polls suggest Sanders would fare better in the general election against Trump, and that he could win. His ideas, such as Medicare for All, free public college education and a $15-per-hour minimum wage, have helped set the parameters of the primary debates and have been embraced by some of his rivals.

For voters seeking the strongest possible repudiation of Trump and his policies, Sanders's focus on economic inequality and climate change offer direct countermeasure against the current president's campaign of tax cuts and withdrawal from the Paris Agreement.

Sanders' aversion to corporate funding is also a part of his appeal. Over many years, suspicion has built up as successive Republican and Democratic administrations, lobbied extensively by corporations and wealthy individuals, embraced neoliberal economic policies that led in turn to increased economic inequality. This stoked the popularity of non-traditional candidates such as Trump. But Trump was himself a benefactor of neoliberal economics, and his administration has embraced corporate interests with alacrity. If the electorate's faith in the presidency has been shaken by Trump, Sanders - another political outsider, albeit from the other end of the spectrum - may benefit from widespread scepticism this would engender.

And while Trump has sought to use identity politics to mobilise the majority to the detriment of social minorities, Sanders has taken aim at a still smaller - but more well-defended - group: those with a net worth of over $32m, whom he has targeted with a significant wealth tax. There are an estimated 607 billionaires in the US, but the impact of their extreme wealth is widely felt.

Sanders is also one of the leading proponents of the Green New Deal, with an ambitious policy proposal to address both climate change and income inequality, in part by creating 20 million new jobs. He has received an A+ rating, the highest rating of any primary candidate, from Greenpeace, and his commitment to the Green New Deal has won him the endorsement of Alexandria Ocasio-Cortez, one of the most popular Democrats with younger voters.

There are obstacles, too. Sanders is 78, and recently suffered a heart attack. And in the US, identifiying as a democratic socialist could be dangerous at the ballot box. In his speech to the UN in September, Trump called socialism the "destroyer of societies"; this was a comment aimed at his base. Many Americans possess a residual Cold War mindset and conflate socialism with communism. Others identify it with authoritarian regimes, such as in Venezuela. While Sanders has stated that his version of socialism does not entail the government owning the means of production, the socialist label is shorter and more easily spread.

If Sanders is to overcome this he will need to educate American about the socialist tradition that defined some of Europe’s most effective and respected leaders such as Clement Attlee, Felipe González, François Mitterrand and Willy Brandt. He will need to show that far from destroying capitalism in their countries, these leaders created wealth and widespread prosperity. To position himself as the political heir to Franklin Roosevelt, whose own New Deal brought America out of the Great Depression, will be difficult, but it would make Sanders the candidate of growth, optimism and opportunity - in stark contrast to the negative, divisive rhetoric of the Trump campaign.

First, however, he has to win the primary, which will once more be a choice between grassroots popularity and the centrist tradition.

Arslan Malik was a member of US secretary of state John Kerry’s policy planning staff during the administration of Barack Obama.