During a news conference on Thursday, Sen. Bernie Sanders (I-Vt.) spoke about his agenda. He also answered questions about how he would campaign against Democratic presidential candidate Hillary Rodham Clinton. (AP)

He seems an unlikely presidential candidate — an ex-hippie, septuagenarian socialist from the liberal reaches of Vermont who rails, in his thick Brooklyn accent, rumpled suit and frizzy pile of white hair, against the “billionaire class” taking over the country.

But there was Sen. Bernie Sanders (I-Vt.) on Thursday launching his campaign for the White House — and representing a challenge to the Democratic front-runner, Hillary Rodham Clinton, as she fights to win over the kind of left-leaning Democrats inclined to heed Sanders’s fiery call to action.

Sanders lifted off his long-shot bid with a news conference outside the U.S. Capitol on Thursday by declaring war on corporate America and billionaire campaign donors. He also landed subtle jabs at Clinton, whose political ties to Wall Street and hawkish worldview have left some liberals yearning for an alternative.

“The major issue is: How do we create an economy that works for all of our people, rather than a small number of billionaires?” Sanders said. Disavowing the Citizens United Supreme Court decision that disrupted the campaign finance system, he added: “We now have a political situation where billionaires are literally able to buy elections and candidates. Let’s not kid ourselves: That is the reality right now.”

As he faces off with Clinton, who is as commanding a favorite for the nomination as any non-incumbent in recent history, Sanders threatens to remind base Democrats why they may be suspicious of her.

Sen. Bernie Sanders (I-Vt.), a White House contender in 2016, is known for his stances on budget issues and war. Here’s the Vermont senator’s take on Obamacare, Social Security and more, in his own words. (Julie Percha/The Washington Post)

The contrast between the two candidates is stark: his authenticity and unvarnished rhetoric to her careful script; his unabashedly liberal agenda to her years of triangulation; his grass-roots campaign to her paid army of staffers and super PAC allies.

Another danger for Clinton: Because of her dominance at the outset, any surge by Sanders or another challenger could be interpreted as a sign of her weakness and erase her aura of inevitability.

Officially, Clinton accepts the challenge. Her allies have long said a competitive primary would make her a stronger nominee in the general election, and her campaign team has been preparing for a real race — against Sanders as well as other likely candidates, including former Maryland governor Martin O’Malley.

Clinton took to Twitter to write: “I agree with Bernie. Focus must be on helping America’s middle class. GOP would hold them back. I welcome him to the race.”

House Minority Leader Nancy Pelosi (D-Calif.), a Clinton supporter, told reporters that she is pleased Sanders is running because “it’s healthy for a party to have an exchange of ideas.” She said more candidates would “enliven the debate, and that will be wholesome.”

Sanders, 73, enters the contest after eight years in the Senate and 16 years in the House. A son of a paint salesman who immigrated from Poland, Sanders has been active in leftist politics since his student days at the University of Chicago. He also served as mayor of Burlington, Vt., in the 1980s.

Sen. Bernie Sanders speaks at a news conference on Capitol Hill on Thursday. Sanders sent out an e-mail earlier to announce that he will run for the White House. (Alex Wong/Getty Images)

On Thursday, Sanders touted his vote opposing the Iraq war in 2002, when he was a House member and Clinton, then a senator from New York, voted to authorize the war. He also highlighted his opposition to an emerging trade deal with a dozen Pacific Rim nations, the initial phases of which Clinton negotiated as secretary of state.

Sanders trained most of his rhetorical fire on David and Charles Koch, the industrialist billionaire brothers whose vast political spending on behalf of Republicans and conservative causes has made them political bogeymen for the left.

But Sanders also suggested that it was valid to raise questions about the Bill, Hillary and Chelsea Clinton Foundation, a nonprofit philanthropy that has come under scrutiny for accepting foreign donations.

Thursday’s event was as unusual as Sanders himself, who evokes an image more in line with a New England professor than a presidential contender. Technically, Sanders had announced his candidacy in an e-mail to supporters earlier in the day, so the news conference was just a chance for him to lay out an agenda.

In a five-minute speech, Sanders neither said that he was running for president nor asked people for their votes. He began his remarks with a “whoa” as the microphone signaled slight feedback, and he took a few questions from reporters.

In Sanders’s recent visits to early caucus and primary states, he has impressed liberal activists. However, he is under no illusions about the challenges ahead.

“We all understand that Hillary Clinton is an incredibly formidable opponent, and beating her in the Democratic nomination process is going to be extremely difficult,” Sanders adviser Tad Devine said. “But I do think there’s a path forward for Bernie.”

That path begins in Iowa, home to the nation’s first presidential caucuses. History is replete with liberal challengers who upset establishment favorites in Iowa, most recently Barack Obama in 2008.

Sanders’s advisers see similarities between Iowa and Vermont: Both are relatively rural states with long traditions of grass-roots organizing. Democrats there also have a populist streak, motivated by issues such as economic fairness and war and peace.

Sanders hopes to do well in New Hampshire, which borders Vermont and hosts the first presidential primary, and in the Nevada caucuses to follow.

Sanders knows he will need to defeat Clinton, at least in a smaller contest, to establish himself as a credible challenger. His strategy is to play aggressively in caucus states, where Clinton performed poorly in 2008, including Colorado and Minnesota. He also sees Massachusetts as a larger primary state that’s winnable, advisers say.

The Sanders campaign, which will be based in Burlington, hopes to raise about $50 million in the primaries to pay for television ads in the early states. Much of that money is expected to come online from the deep network of small-dollar donors Sanders has built over the years. As he has joked, “I do not have millionaire or billionaire friends.”

As of now, there is no official pro-Sanders super PAC. But Sanders hopes to use that absence to his advantage, making super PAC spending a centerpiece of his populist message. This could be potent, especially in Iowa and New Hampshire, where voters may grow exhausted by the onslaught of political television advertising over the next year.

Sanders is most comfortable campaigning in town-hall settings, as opposed to reading speeches from teleprompters, which his advisers cite as a strength in early states.

“He’s very real; he’s very good just interacting and talking and being himself,” Devine said. “In this age, when voters are really into authenticity, it’s just a better way to present a candidate.”

Addressing a scrum of television cameras from a grassy spot outside the Capitol known as “The Swamp,” Sanders remarked that the nation was “looking at a guy indisputably who has the most unusual political history of anybody in the United States Congress.”

He is the longest-serving independent in Congress, first winning a House seat in 1990, and has refused to formally join the Democratic Party, although he has caucused with Democrats in both chambers.

Even now, as he seeks the party’s highest calling, its presidential nomination, Sanders rejected any suggestion that he register as a Democrat.

“No,” he said, stepping away from the news conference, “I’m an independent.”