When Senator Bernie Sanders announced that he was seeking the Democratic nomination, few people noticed.

Senator Sanders represents Vermont, a state with the second-smallest population in the United States. He is not even a registered Democrat, but an Independent, in Congress.

Senator Sanders has white hair. And he hunches his shoulders and his body when he walks and speaks to a crowd. There is a slightly rumpled look about him. At 74 years old, Bernie Sanders is also the oldest candidate for president.

But he seems to get bigger by the day. He fills large halls and stadiums. Nearly 30,000 people went to see him in Portland, Oregon; 27,000 in Los Angeles; 15,000 in Seattle; and 11,000 in Phoenix, Arizona. His supporters use the phrase "#FeelTheBern" to promote him. It is a play on the word "burn" that sounds like "Bern," and means something is on fire.

And they are giving him money, plenty of it.

Bernie Sanders raised almost as much money as front-runner Hillary Clinton in the past three months. She raised $28 million. He brought in $26 million, mostly in small donations from individuals.

David Axelrod is a former advisor for President Obama. He tweeted that "Sanders fundraising is remarkable." He added, it is "very possible" that Mr. Sanders has more money available than Ms. Clinton.

Kyle Kondick is with the University of Virginia Center for Politics. He says Bernie Sanders appeals to Democratic voters for at least two reasons.

"And so he is, one, he's not Hillary Clinton and two, he's more liberal than Hillary Clinton, and there is certainly a desire for that amongst at least some Democratic voters."

Most political observers, including Mr. Kondick, say Mr. Sanders probably cannot win the Democratic nomination. They say he is too liberal, too far to the left politically, and too old to win.

Early in the campaign, he had problems with supporters from the #BlackLivesMatter movement. That movement fights against racism, especially how police sometimes respond differently to darker skinned people than lighter. Since then he has reached out to black voters, a key voting group.

In recent months, Senator Sanders has climbed in the polls. They show him leading in New Hampshire, a small, northeastern American state next to Vermont. It will hold the first primary in the nation on February 9, 2016.

But across the nation, support for Hillary Clinton is still 16 points higher in the polls.

The process of nominating a candidate in the U.S. is complex. Not everyone in the U.S. understands how it works. About 4,500 delegates will attend the Democratic National Convention next July.

Among them are 700 party leaders, also known as "super-delegates." The party leaders have more influence in the process to nominate a candidate for president. Ms. Clinton's campaign says she has the endorsement of most of those party leaders.

Mr. Kondick explains why Democratic Party leaders support Ms. Clinton:

"Hardly any of those people [party leaders] support Sanders because they don't think that he could win a general election, and they think he's too liberal."

However, those super-delegates can change their votes up to the last minute of voting at the convention.

Why do people support Sanders?

Paul Heintz is the political editor for Seven Days newspaper in Senator Sanders' hometown of Burlington, Vermont. He has covered Bernie Sanders for years. Like many people, he says Mr. Sanders is doing "far better" in this race than expected.

Mr. Heintz says not all Democrats favor Hillary Clinton. Critics question why she used a private, unofficial email server when she was Secretary of State.

Many people like Mr. Sanders' economic message, says Paul Heintz. Americans are still hurting from the 2008 financial crisis that caused unemployment and loss of assets.

"And though the number of jobs are coming back, the amount that people are being paid hasn't quite come back. So a number of people are feeling, many people, are feeling a tremendous economic insecurity. And Senator Sanders message has always been focused on that."

Authentic is a word often used to describe Mr. Sanders. People say he is "authentic," which means real, and not a fake personality.

Kyle Klondick says that Bernie Sanders lacks the same style or positive messaging of successful past presidential candidates — candidates like Ronald Reagan, Bill Clinton and Barack Obama.

"Sanders' speaking style is very blunt, very direct and his stump speech is almost just this big criticism of the country and how we have problems with income inequality and other issues of fairness that are legitimate points to make, but I think that his speaking style is frankly kind of a downer."

Campaigning as an outsider

Bernie Sanders, like Republican front-runner Donald Trump, has talked about improving the economy. Mr. Trump says he would cut taxes for wealthy people and businesses. Mr. Sanders says he would raise taxes on them. Both have captured attention by running as outsiders.

While Bernie Sanders appears to be an outsider, he has had a long political career. It started when he was elected mayor of Burlington, Vermont in 1981, getting 10 votes more than his opponent.

In 1990, he was elected to the House of Representatives, representing Vermont. As a registered Independent, he did not belong to either party and had few successes. Mr. Heintz says:

"In his time in Congress, he has never really been a major player. And that's in part because he hasn't been a member of the Democratic Party, and he hasn't played by the conventional rules."

He was elected to the Senate in 2006. As chairman of the Veterans Affairs Committee, he worked with Republican Senator John McCain to improve health care for military veterans.

Bernie Sanders grew up in an apartment in Brooklyn, New York. He is Jewish, but says he does not follow the religion closely. If elected president, he would be the first Jew elected to the highest office in the U.S.

Debate to test Sanders' appeal

On Tuesday night, Bernie Sanders and Hillary Clinton will debate with three other Democratic candidates. The news agency CNN hosts the debate in Las Vegas. The TV network says Mr. Sanders "will face his biggest test yet: going toe-to-toe with the former Secretary of State."

The Sanders campaign says he is studying different subjects. But he is not rehearsing for the debate as other candidates usually do. With a national audience, Mr. Sanders is ready to discuss free college tuition, health care, and a $15-an-hour minimum wage. And he will need to answer how to pay for these programs.

The debate also gives Mr. Sanders a chance to compare his positions against Ms. Clinton's. Supporters for Mr. Sanders use social media to show the differences between the candidates and their issues. Those include the death penalty and the U.S. government's bail out of financial institutions, but not average citizens.

With recent mass shootings in the U.S., Senator Sanders is sure to be questioned about his support of gun owners. Vermont is a rural state where many people own guns for hunting and recreation. He used to favor more gun rights earlier in his career. Now he favors more gun control.

Outside the debates, a wild card is possible: Vice President Joe Biden. He is the man missing from the debate stage. If the well-liked and experienced Mr. Biden announces that he will run, the race could change considerably.

I'm Anne Ball.