PHOTOGRAPH BY DANIEL ACKER/BLOOMBERG VIA GETTY

Six days after formally entering the 2016 Presidential race, Senator Bernie Sanders is having some time of it. After attracting overflow crowds at a number of stops in Iowa late last week, Sanders moved on to Minnesota on Sunday, where he appeared at the Minneapolis American Indian Center and declared, “Our country belongs to all of our people and not just a handful of billionaires.” According to a report in the Minneapolis Star Tribune, about three thousand people turned out. A local television station estimated the number of attendees at four thousand.

Whatever the exact number was, the seventy-three-year-old from Vermont appears to be attracting bigger crowds than any other candidate, Democrat or Republican. With one recent national survey finding that fifteen per cent of likely Democratic voters support him, and a new Des Moines Register poll showing him picking up sixteen per cent of the Democratic vote in Iowa, the media is starting to accord him some serious attention. The Times, having initially failed to report Sanders’s formal announcement of his candidacy in its print edition, ran a front-page story on Friday about his appeal to senior citizens, and another piece over the weekend about the enthusiastic reception he was receiving in Iowa. Before speaking in Minneapolis on Sunday, Sanders appeared on NBC’s “Meet the Press,” where he highlighted the “grotesque level of income and wealth inequality” in the U.S. and said, “I think we need a political revolution in this country.”

To be sure, not all of the Sanders coverage has been helpful to his campaign. Last week, Mother Jones, as an accompaniment to an interesting piece about his early years in left-wing politics, reprinted an article he wrote in 1972 for an alternative newspaper called the Vermont Freeman, the subject of which was male and female power relations, and sexual fantasies. One line in particular got quite a bit of media attention, including an explainer by NPR’s Danielle Kurtzleben. “A woman enjoys intercourse with her man—as she fantasizes being raped by 3 men simultaneously,” Sanders wrote. On “Meet the Press,” the host, Chuck Todd, asked the senator about the piece. Sanders described it as fiction, adding, “It was dealing with gender stereotypes, why some men like to oppress women, why other women like to be submissive. You know, something like ‘Fifty Shades of Grey.’ Very poorly written, forty-three years ago.”

While it is embarrassing to Sanders, the flap about his literary effort, which he wrote while trying to get by as a carpenter and freelance journalist, seems unlikely to have much lasting effect on his campaign. That’s partly because Sanders’s run for the White House isn’t based on his personal character, or even his record as a mayor, congressman, and U.S. senator. Sanders is running for a cause—a resurgent progressivism that was conceived during decades of wage stagnation and rising inequality, born during the great financial crisis of 2008, and announced on the political stage by the street protests of the Occupy Wall Street movement and the widespread public support they engendered.

Of course, many candidates claim that their campaigns aren’t about them but about something larger. But when Sanders uses this line, which he does all the time, he is merely stating a fact. Before he entered the 2016 race, the new progressivism was a cause in search of a candidate. After Senator Elizabeth Warren refused to step up and run, Sanders seized his chance, and he is now getting his reward. In a contest dominated by a consummate insider with strong ties to the moneyed élite, many disaffected Democrats are embracing him as an underdog and an outsider. During a general election, almost all of Sanders’s supporters would vote for Clinton over Jeb Bush or any other Republican, but, right now, his presence in the primary gives them the opportunity to raise a rumpus, and to try to pull the party in a liberal direction.

As the campaign progresses, it will be fascinating to see how far this effort succeeds. Already, Clinton has shifted her stance on immigration reform and the criminal-justice system. In two recent speeches, she pledged to extend President Obama’s initiatives aimed at undocumented workers and their families, and called for an end to mass incarceration.

Although each of these policy proposals is important in its own right, neither would cost the Democratic Party’s donor class any money. The political test for Clinton will come in the area of economic policy, where Sanders has put out a comprehensive and, by American standards, quite radical manifesto. It includes reforming the tax code to make the rich pay more, raising the federal minimum wage to fifteen dollars an hour, reforming trade policies, breaking up the big banks, and turning Medicare into a public health-care system for Americans of all ages.

When Sanders unveiled this plan last December, I pointed out that it isn’t all at variance with the policies of the Clinton-Obama wing of the party. Virtually all Democrats support raising the minimum wage and eliminating some of the tax breaks for the rich, for instance. In these areas, and others, Clinton should be able to find common ground with progressives. But the question remains: In positioning herself as a battler for the middle class, how far to the left will she go?

We may get some clues on June 13th, when she speaks at Franklin D. Roosevelt Four Freedoms Park, on Roosevelt Island. In a note to reporters on Monday, a campaign official said that Clinton would “lay out her view of the challenges facing this country and her vision and ideas for moving the country forward.” As I wrote on Friday, Clinton has made a strong start in the opinion polls, with her approval ratings largely withstanding the barrage of stories about deleted e-mails and the finances of the Clinton Foundation. But with Sanders and Martin O’Malley, the former governor of Maryland, now officially in the race, the dynamics are changing. Although neither of Clinton’s challengers is known as a political cutthroat, both will look to knock holes in Clinton’s record, and to draw contrasts with their own policy positions.

Sanders, indeed, has already begun. “I know where I have been on trade agreements,” he told NBC’s Todd. “I know where I have been on Wall Street. I know where I have been on the Keystone pipeline. And Secretary Clinton will obviously explain her position to the American people.”