Mere months ago, the thought of anyone posing a serious threat to Hillary Clinton's Democratic presidential campaign was wishful thinking at best.

But with the Iowa caucus just weeks away, here comes Vermont Senator Bernie Sanders, the self-described "democratic socialist," making what looks to be a real competitive run for the nomination.

"I think you still have to consider [Clinton] as the prohibitive favourite or the strong favourite," says Sean Trende, a senior elections analyst for RealClearPolitics.com.

"You cannot look at the numbers in Iowa and New Hampshire and nationally and not think 'OK, maybe it's going to happen again.'"

The reference, of course, is to the 2008 nomination campaign when an unproven one-term senator, Barack Obama, came from behind and went on to defeat the better known Clinton in the primaries.

Whether the 74-year-old Sanders, who has been drawing large enthusiastic crowds to his campaign events recently, can repeat this feat is unclear.

Recent polls show he's competitive in Iowa, leading in New Hampshire, the next-door state to his native Vermont, and polling within single digits of Clinton nationally.

Like a certain New York real estate magnate, he has been able to tap into a part of a dissatisfied electorate, in his case attracting mostly working-class Americans who feel that the system is rigged against them.

Authenticity

Garrison Nelson, a University of Vermont political science professor who has known Sanders for 40 years, says his basic appeal boils down to one word: authenticity.

"I've heard him make the same arguments for 40 years. He has not differed a whit," Nelson said. "And at a time when people are tired of politicians making promises that they cannot fulfil and taking stances they abandon once elections are over, having someone like Bernie Sanders out there is refreshing."

That authentic label is one that could also account in part for the broad support of Republican presidential candidate Donald Trump. But there are key differences. Trump's base of discontented voters seems to encompass those targeting the weaker end of the spectrum — like poor immigrants from Mexico, Nelson says.

Sanders has been drawing large, enthusiastic crowds to his campaign events. (John Minchillo/Associated Press)

Meanwhile, Sanders supporters target those at the upper end: the millionaires and billionaires who he and his followers believe have enriched themselves over the past 20 years with no discernable change in the financial well-being of the middle and working classes.

Sanders's attacks on Wall Street have made him a champion of the progressive wing of the Democratic party, those who believe that the principal problem in the economy is not growth but inequality, says William A. Galston, a senior fellow at the Brookings Institute and a former policy adviser to Bill Clinton.

"It is very clear that Sanders has found a real audience with … the sorts of people who hoped for more from Barack Obama than they got," Galston said.

"Obama, in the end, did not emerge as the kind of class warrior that a lot of people wanted, and Bernie Sanders is the closest thing to a class warrior that we've seen in a very long time."

'Vanilla socialist'

As Nelson puts it, Sanders is somewhat of a political phenomenon, a left-wing evangelical, who has carved out success in a country where describing oneself as a liberal is often considered political suicide.

But he describes Sanders as more of a "vanilla socialist" who is different from other socialists because "he wants to win."

Sanders was never a hippie, said Nelson, nor a 1960s social or cultural radical. Instead, his politics are more akin to the 1930s and 40s socialism of trade unions and president Franklin D. Roosevelt's New Deal.

His agenda is mostly a laundry list of liberal causes: tax reforms that target corporations, more aggressive programs to redistribute wealth, tighter regulations on Wall Street, opposition to trade deals like NAFTA and the Trans-Pacific Partnership, free tuition for public colleges and universities, and support of universal health care.

"I don't think Bernie Sanders is a democratic socialist. He's a social democrat," said Galston. "The programs he's advocating would be perfectly recognizable in the Nordic countries. He's a northern European social democrat of a type that I think Canadians have more experience with than Americans do."

At the same time, Sanders has also drawn praise from some Republicans on issues such as immigration reform, as members from both camps fear allowing more guest workers in will suppress the wages of U.S. workers.

On gun control, as well, he has been criticized by gun control advocates for his mixed record, which includes voting against the so-called Brady Bill, one of the earlier attempts at reform.

His political career began in Burlington, Vt., where he was elected mayor in 1981 by just 10 votes. He was later elected eight times to the U.S. Congress and twice to the Senate, all as an independent, making him the most successful independent U.S. politician of all time.

He's a terrific street campaigner, say Garrison, yet his public demeanour is somewhat prickly, and he's not exactly known for shaking hands and kissing babies.

"He's going to discuss issues with you," Nelson said. "I've watched him multiple times, people yelling at him. And rather than walk away, he would go up to them and ask them to explain themselves."

Democratic presidential candidate Hillary Clinton has a bigger advantage this time around, compared to the 2008 primary campaign, in which she lost to Barack Obama. (Patrick Semansky/Associated Press)

Still, while he may be attracting big crowds at the moment, and surging in the polls, he still faces many challenges in pulling off an Obama-like victory over Clinton.

In 2008, Clinton had more establishment support than Obama but didn't have close to majority support from elected Democratic officials in Congress or state governors, noted Harry Enten, a senior political analyst for the website FiveThirtyEight.com

"Now, she has the most support from these officials pre-Iowa caucus of any non-incumbent presidential candidate in the modern era," he said.

Sanders's support from white progressive males

The support for Sanders is mostly coming from white progressive males, and he tends to poll strongest in states with large white populations.

"That, of course, matches Iowa and New Hampshire to a T," Enten said.

For Sanders, the problem is that, after Iowa and New Hampshire, he will soon run into those Super Tuesday states that are in the South, heavily minority and strongly in favour Clinton, added Trende.

The question also becomes whether a politician holding such left-leaning views could win the presidency.

"There's not a socialist majority in the U.S," said Galston. "There's not even a majority for the Europeanization of the American social welfare state.

"American political culture defaults to mistrust of government, and right now, mistrust of government is at a peak. And so here's Senator Sanders coming along saying, 'You mistrust government now? I'm going to give you a much bigger government to mistrust.'"