Senator Bernie Sanders’ campaign to become the Democratic Party’s candidate for president has broken into the mainstream. He is consistently drawing far larger crowds than any other aspirant in either the Democratic or Republican parties.

Polls show his support is climbing, and in one state, New Hampshire, he has moved ahead of Democratic front-runner Hillary Clinton. He may win some states in the Democratic primaries.

What is striking about the Sanders phenomenon is that his main message echoes economic themes made popular by the Occupy movement of 2011. These include growing inequality in the United States and the corporate hijacking of American politics. He raises the issue of climate change and takes many pro-worker positions.

Sanders is running as an open socialist. This puts socialism and the issues he raises back into the wider political debates.

A discussion has opened within and between socialist organisations about how best to relate to this opportunity. On one side are those who have joined Sanders’ bid to become the Democratic Party’s presidential candidate, and on the other those who argue against that course and are working to win Sanders’ supporters to break with the Democratic Party.

This is an old debate on the left in the US. It is not about tactics, but about strategy: class collaboration or class struggle.

The largest revolutionary socialist group, the International Socialist Organization (ISO), has taken the lead in explaining the trap of supporting Sanders’ campaign.

Some trade union activists and socialists have formed Labor for Bernie. Its founding statement concludes: “We call on labor leaders, union members and working people to unite behind Bernie Sanders for a voice in the presidential political process and to elect the president working families need – a president who will answer to the 99 percent!”

ISO leader Ashley Smith writes of the left support for Sanders: “In running for the Democratic presidential nomination as the liberal outsider with almost no chance of winning, Sanders isn’t very ‘bold’ … By steering liberal and left supporters into a Democratic Party whose policies and politics he claims to disagree with, Sanders – no matter how critical he might be of Hillary Clinton – is acting as the opposite of an ‘alternative’ …

“If Sanders had his heart set on national politics, he could have run for president like Ralph Nader [did in 2000] as an independent opposing both capitalist parties … He would have been appealing for a protest vote, rather than a real chance to win, but Sanders rejected this possibility out of hand for a different reason. ‘No matter what I will do,’ Sanders said [when he announced his campaign], ‘I will not be a spoiler’.”

In 2000, Nader won nearly 3 million votes against the capitalist parties. The Democrats and reformist socialists roundly attacked him as a “spoiler” who helped George Bush win. Sanders proclaims, and his campaign loudly echoes, that he will support whoever is the Democratic Party nominee.

Sanders vehemently attacked Nader’s candidacy. Before he decided to run, Sanders said, “If I decide to run, I’m not running against Hillary Clinton. I’m running for a declining middle class”.

Sanders has appointed as his campaign manager Ted Divine, a quintessential corporate Democratic Party insider.

Smith accurately describes the dynamic of the Sanders campaign, as against the wishful thinking of some of Sanders’ socialist supporters. “The Democratic Party establishment can breathe a collective sigh of relief. It doesn’t, in fact, fear liberal Democrats like … Sanders, but third party challenges like Nader’s that have the prospect of breaking their stranglehold on votes from workers and the oppressed …

“Hillary Clinton certainly doesn’t regard Sanders as a threat … ‘I agree with Bernie’, she wrote on Twitter. ‘Focus must be on helping America’s middle class. GOP would hold them back. I welcome him to the race.’

“You can expect that Clinton will agree with Sanders during the campaign, rearticulating some of his themes in a ‘more realistic’ fashion and occasionally chiding him for taking things too far …

“At this stage, Clinton is the overwhelming favorite to emerge as the Democratic nominee. If she stumbles in some irreversible way, the corporate establishment that controls the Democratic Party will come up with another more mainstream candidate, like Obama in 2008. Either way, the eventual Democratic presidential nominee will toe the capitalist line.

“However much he disagrees with that candidate, Sanders will agitate for trade unionists and social movement activists to vote for the lesser of two evils. The result is that he will help corral people on the left from taking any steps toward building a genuine alternative to the two-party status quo.”

This is the main reason for not supporting Sanders’ campaign: it crosses the class line, a matter of strategy.

Most socialists disagree with Sanders’ social democratic politics on many issues – his support of US foreign policy, his support of Israel’s wars, his reactionary stand on immigrants and so forth. If he were running as an independent socialist, it might be possible to call for a vote for him while vigorously raising these differences. But that is not the case.

There is, however, one issue that should give socialist supporters of Sanders pause. That is his insulting behaviour toward the Black Lives Matter movement. Two young African American women interrupted one of Sanders’ rallies to talk about the police murders of Blacks.

Sanders responded by lecturing them, explaining that they had nothing to teach him because he has been an anti-racist for 50 years. Recently, two Black Lives Matter activists also interrupted a Sanders speech on the first anniversary of the police murder of Michael Brown in Ferguson, an anniversary that Sanders ignored. These instances received wide publicity. Hillary Clinton has moved to the left of Sanders on this.

He is now trying to walk back on the issue, hiring an African American to be part of his campaign, and modifying his web site a bit. But right now he looks like a white radical who doesn’t understand the most important social movement in the US. Socialists who support him are in danger of being tarnished with the same brush.