No one asked Bernie Sanders what he thought about the Greek referendum on Sunday, but he shared his thoughts anyway.

“I applaud the people of Greece for saying ‘no’ to more austerity for the poor, the children, the sick and the elderly,” Sanders said in welcoming Sunday’s vote, even as it rattled world markets and provoked predictions of economic doom. The statement didn’t just align Sanders with left-wing Europeans; it aligned him with lefter-wing Greek socialists who are too radical for some of those left-wing Europeans.

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Democratic primaries have always featured liberal insurgent candidates, but perhaps none quite so liberal or insurgent as the socialist senator from Vermont. Sanders’ comments are a reminder of just how far the second-place Democratic presidential candidate stands from the American mainstream on some issues, and the looming reckoning Democrats face with their party’s leftward drift.

Never mind whether Sanders can crack 40 percent in any primary against Hillary Clinton — he has already established himself as her de facto challenger and a standard-bearer of a party that was, until this year, too far to the right for his liking.

“When I hear Bernie talk I’m almost inclined to accuse him of plagiarizing me,” said Ralph Nader, the left-wing gadfly whose third-party bid many Democrats still blame for swinging the 2000 election to George W. Bush.

Nader’s kinship with Sanders is yet another sign that the Democratic Party’s goal posts have moved left. The percentage of Democrats who identify as socially and economically liberal has increased 17 points since 2001, according to a recent Gallup poll. And the party’s restive liberal base — led in recent years by progressive icon Massachusetts Sen. Elizabeth Warren — has pushed the party establishment on social issues like same-sex marriage and populist economic ones like equal pay and paid sick leave.

Republicans, at least, are betting that broad swaths of the electorate have been left behind.

“Look, Barack Obama and Hillary Clinton and the other Democrats in D.C., they’re for socialism. They just — they’re not as honest as Bernie is,” said Louisiana Gov. Bobby Jindal on Hugh Hewitt’s radio show last week. “Did you honestly believe we would live to see the day when a guy, an honest-to-goodness socialist, is running for president and doing — he got 10,000 people in Wisconsin to come hear him speak, he’s gaining on Hillary in the polls? This shows you how radical the Democratic Party is.”

In Wisconsin, where Sanders appeared last week before a progressive throng in Madison, the state Republican Party put up billboards featuring Sanders and Clinton riding together on a moped with the words “Left and Lefter” and “extreme policies.” Shortly before Sanders’ speech, Gov. Scott Walker took the unusual step of criticizing the long-shot Sanders, saying his approach is in “stark opposition to most Americans.”

It’s usually Democrats who play this game — as they did with Republican challengers to Mitt Romney in 2012, or with fringe characters like Todd Aiken. Now, it’s Republicans seeking to use the Sanders surge to portray Democrats as radical and out of touch.

And that’s making many Democrats nervous, said Joe Trippi, who ran Vermonter Howard Dean’s campaign in 2004.

“We can’t lose the presidency. We can’t take a risk by nominating somebody outside the comfort zone. That’s what’s driving the inevitable-ness” of Clinton, said Trippi, speaking about the party establishment’s thinking.

Sanders is unlikely to tone it down for the long-term good of his newly adopted party.

“Bernie is saying what he believes. He’s unlikely to run for president again, and this is his shot … This is as unfiltered and as clear as it comes,” said liberal labor economist Robert Reich, who compared Sanders to past Democratic candidates like Eugene McCarthy and George McGovern.

But Sanders comes from an even more radical milieu than those left-wing insurgents of the 1960s and 70s.

As mayor of Burlington, Vermont, in the 1980s, Sanders visited Nicaragua in solidarity with its socialist Sandinista regime and later honeymooned in the Soviet Union, where he established a sister-city relationship with the community of Yaroslavl. In the mid-1960s, when Clinton was calling herself a “Goldwater girl,” Sanders spent time on a kibbutz in Israel.

“I think he’s the most leftist, and I think he is the greatest megaphone for leftist dissent” since Henry Wallace in 1948, said Doug Wilson, who served as deputy campaign manager for Gary Hart in 1984, who challenged establishment front-runner Walter Mondale. (Wilson and Hart are backing former Maryland Gov. Martin O’Malley in the Democratic primary.)

And Sanders has long been unabashed about his socialist beliefs. “Nobody should earn more than $1 million,” he told the Burlington Free Press in 1974.

“I believe that, in the long run, major industries in this state and nation should be publicly owned and controlled by the workers themselves,” he wrote in 1976.

While statements like these might be a skeleton in another candidate’s closet, Sanders has never renounced socialism, even if his brand of it has become more moderate. “Do they think I’m afraid of the word? I’m not afraid of the word,” Sanders said of the term “socialist” in an interview with The Nation on Monday.

Sanders — pointing to high approval numbers for a higher minimum wage, pay equity for women and other issues — often argues his agenda is mainstream. “It is not a radical agenda,” he said at a breakfast for reporters last month. “In virtually every instance, what I’m saying is supported by a significant majority of the American people.”

Still, many of his most important positions today fall well outside the traditional parameters that have bounded American political discourse in recent decades. He wants to raise the marginal tax rate for top earners to more than 50 percent — which would be the highest rate in 30 years and is more than 10 points higher than Barack Obama proposed as a candidate in 2007. He says he would replace the Affordable Care Act — perhaps Obama’s signature accomplishment in office and a prized victory for Democrats — with a Medicare-for-all, single-payer system, a position that was too liberal for Dean when he was governor of Vermont. (Sanders’ standard Obamacare line is to quickly laud its “modest gains” but quickly say it isn’t enough.)

He has called for free tuition at four-year public colleges and universities and introduced legislation that would break up banks like Bank of America, JPMorgan Chase, Goldman Sachs and Citigroup. He said this weekend that he’d consider left-wing New York Times columnist Paul Krugman for his Cabinet.

But if all of that goes too far for the general American public, much of the Democratic Party is right there with him. “He’s not in the mainstream of the Democratic apparatchiks and the paymasters,” said Nader, “but look at where he polls on breaking up the big banks, on opposing trade dictatorships.”

Michael Kruse and Brian Faler contributed to this report.