(CNN) Bernie Sanders' stellar victory in New Hampshire showed that his campaign has some serious legs. He won 6 of 10 voters and led in virtually every demographic category, except for those over 65 and households earning more than $200,000.

Slowly, Sanders has been undercutting Hillary Clinton's electoral strength and putting together a campaign that could go all the way into the summer convention.

The Clintons continue to dismiss the idea that Sanders is a serious candidate, painting him as someone who is too left of center and a politician who doesn't make the kind of serious, detailed arguments that most adult voters will demand. To be sure, there is a good deal of logic to the Clinton argument. It is extraordinarily difficult to imagine that a democratic socialist could win a general election in the post-Ronald Reagan era.

Biden greets Sanders with an elbow bump before the start of a debate in Washington in March 2020. They went with an elbow bump instead of a handshake because of the coronavirus pandemic.

Biden greets Sanders with an elbow bump before the start of a debate in Washington in March 2020. They went with an elbow bump instead of a handshake because of the coronavirus pandemic.

Sanders speaks to reporters in Burlington, Vermont, a day after Super Tuesday II. Sanders said it "was not a good night for our campaign from a delegate point of view" but that he looked forward to staying in the race and taking on Joe Biden in an upcoming debate.

In a tense and dramatic exchange moments after a Democratic debate, Warren accused Sanders of calling her a liar on national television. Sanders responded that it was Warren who called him a liar. Earlier in the debate, the two disagreed on whether Sanders told Warren, during a private dinner in 2018, that he didn't believe a woman could win the presidency.

Sanders campaigns at the University of New Hampshire in September 2019. A few days later, he took himself off the campaign trail after doctors treated a blockage in one of his arteries. Sanders suffered a heart attack, his campaign confirmed.

Sanders looks at his notes as he watches President Trump deliver the State of the Union address in February 2019. That month, Sanders announced that he would be running for president again.

Sanders looks at his notes as he watches President Trump deliver the State of the Union address in February 2019. That month, Sanders announced that he would be running for president again.

Sanders brings a giant printout of one of Donald Trump's tweets to a Senate debate in January 2017. In the tweet, Trump had promised not to cut Social Security, Medicare and Medicaid.

Sanders speaks at a rally in Santa Monica, California, in June 2016. He pledged to stay in the Democratic race even though Clinton secured the delegates she needed to become the presumptive nominee.

Sanders speaks at a rally in Santa Monica, California, in June 2016. He pledged to stay in the Democratic race even though Clinton secured the delegates she needed to become the presumptive nominee.

Sanders speaks at a campaign rally in Ann Arbor, Michigan, in March 2016. He won the state's primary the next day, an upset that delivered a sharp blow to Clinton's hopes of quickly securing the nomination.

Sanders and his wife, Jane, wave to the crowd during a primary night rally in Concord, New Hampshire, in February 2016. Sanders defeated Clinton in the New Hampshire primary with 60% of the vote, becoming the first Jewish candidate to win a presidential primary.

Comedian Larry David and Sanders appear together on "Saturday Night Live" in February 2016. David had played Sanders in a series of sketches throughout the campaign season.

Sanders sits with rapper and activist Killer Mike at the Busy Bee Cafe in Atlanta in November 2015. That evening, Killer Mike introduced Sanders at a campaign event in the city. "I'm talking about a revolutionary," the rapper told supporters. "In my heart of hearts, I truly believe that Sen. Bernie Sanders is the right man to lead this country."

Sanders embraces Remaz Abdelgader, a Muslim student, during an October 2015 event at George Mason University in Fairfax, Virginia. Asked what he would do about Islamophobia in the United States, Sanders said he was determined to fight racism and "build a nation in which we all stand together as one people."

Sanders embraces Remaz Abdelgader, a Muslim student, during an October 2015 event at George Mason University in Fairfax, Virginia. Asked what he would do about Islamophobia in the United States, Sanders said he was determined to fight racism and "build a nation in which we all stand together as one people."

Seconds after Sanders took the stage for a campaign rally in August 2015, a dozen protesters from Seattle's Black Lives Matter chapter jumped barricades and grabbed the microphone from the senator. Holding a banner that said "Smash Racism," two of the protesters -- Marissa Johnson, left, and Mara Jacqueline Willaford -- began to address the crowd.

In July 2015, two months after announcing he would be seeking the Democratic Party's nomination for President, Sanders spoke to nearly 10,000 supporters in Madison, Wisconsin. "Tonight we have made a little bit of history," he said. "You may know that some 25 candidates are running for President of the United States, but tonight we have more people at a meeting for a candidate for President of the United States than any other candidate has."

Sanders and US Rep. Jeff Miller, chairman of the House Committee on Veterans' Affairs, walk to a news conference on Capitol Hill in 2014. Sanders was chairman of the Senate Committee on Veterans' Affairs.

Sanders and US Rep. Jeff Miller, chairman of the House Committee on Veterans' Affairs, walk to a news conference on Capitol Hill in 2014. Sanders was chairman of the Senate Committee on Veterans' Affairs.

Sanders speaks to reporters in 2010 about the Obama administration's push to extend Bush-era tax cuts. Three days later, Sanders held a filibuster against the reinstatement of the tax cuts. His speech, which lasted more than eight hours, was published in book form in 2011. It is called "The Speech: A Historic Filibuster on Corporate Greed and the Decline of Our Middle Class."

Sanders speaks to reporters in 2010 about the Obama administration's push to extend Bush-era tax cuts. Three days later, Sanders held a filibuster against the reinstatement of the tax cuts. His speech, which lasted more than eight hours, was published in book form in 2011. It is called "The Speech: A Historic Filibuster on Corporate Greed and the Decline of Our Middle Class."

Sanders chats with Dr. John Matthew, director of The Health Center in Plainfield, Vermont, in May 2007. Sanders was in Plainfield to celebrate a new source of federal funding for The Health Center.

Sanders chats with Dr. John Matthew, director of The Health Center in Plainfield, Vermont, in May 2007. Sanders was in Plainfield to celebrate a new source of federal funding for The Health Center.

Sanders sits next to President Bill Clinton in 1993 before the Congressional Progressive Caucus held a meeting at the White House. Sanders co-founded the caucus in 1991 and served as its first chairman.

Sanders sits next to President Bill Clinton in 1993 before the Congressional Progressive Caucus held a meeting at the White House. Sanders co-founded the caucus in 1991 and served as its first chairman.

Sanders reads mail at his campaign office in Burlington in 1990. He was running for the US House of Representatives after an unsuccessful bid in 1988.

Sanders reads mail at his campaign office in Burlington in 1990. He was running for the US House of Representatives after an unsuccessful bid in 1988.

In 1987, Sanders and a group of Vermont musicians recorded a spoken-word folk album. "We Shall Overcome" was first released as a cassette that sold about 600 copies. When Sanders entered the US presidential race in 2015, the album surged in online sales. But at a CNN town hall, Sanders said, "It's the worst album ever recorded."

Sanders takes the oath of office to become the mayor of Burlington, Vermont, in 1981. He ran as an independent and won the race by 10 votes.

Sanders takes the oath of office to become the mayor of Burlington, Vermont, in 1981. He ran as an independent and won the race by 10 votes.

Sanders, right, leads a sit-in organized by the Congress of Racial Equality in 1962. The demonstration was staged to oppose housing segregation at the University of Chicago. It was Chicago's first civil rights sit-in.

Sanders, right, leads a sit-in organized by the Congress of Racial Equality in 1962. The demonstration was staged to oppose housing segregation at the University of Chicago. It was Chicago's first civil rights sit-in.

US Sen. Bernie Sanders speaks at a campaign rally in Chicago in March 2019. Sanders, an independent from Vermont, is the longest-serving independent in the history of Congress.

US Sen. Bernie Sanders speaks at a campaign rally in Chicago in March 2019. Sanders, an independent from Vermont, is the longest-serving independent in the history of Congress.

But Sanders' political appeal is based on much more than the thrill of an anti-establishment insurgent or some unexpected love affair of millennials with a Brooklyn socialist. Part of what has given Sanders his strength is how mainstream many of his standard political arguments are. If one listens to what he has been saying, it is possible to see that Sanders is not that radical at all. In many respects, his campaign directly addresses fundamental concerns that a wide range of Americans have about their future.

Corrupt political system

The best known issue in Sanders' arsenal is the claim that there is too much money in politics. The government is constantly unable to respond to the concerns of many Americans, not because the parties don't like each other or because the mainstream media creates a destructive environment, but because big interest groups and lobbyists have disproportionate power in Washington as a result of their donations. In their landmark book, "Winner Take All Politics," the political scientists Jacob Hacker and Paul Pierson provided a powerful account of how the growth of corporate lobbies in the 1970s produced changes in public policy that greatly worsened inequality.

The breakdown of the post-Watergate campaign finance system, culminating with the Supreme Court's Citizens United decision, has produced a political process where there are almost no barriers to flooding politicians with dollars.

Without reforming this process, it is unrealistic to expect that any president or Congress will be able to enact substantive changes that challenge the status quo. It makes sense when he says that, "American democracy is not about billionaires being able to buy candidates and elections. It is not about the Koch brothers, Sheldon Adelson and other incredibly wealthy individuals spending billions of dollars to elect candidates who will make the rich richer and everyone else poorer." One only has to read Jane Mayer's brilliant new history of the Koch brothers, "Dark Money," to understand why this argument is so important.

When Sanders speaks about this issue with passion and principle, in way that is difficult for Hillary Clinton (given her ties to big money through fund raising, speech-making and the Clinton Foundation), his arguments ring true with many Americans, including centrists, who feel this is a fundamental issue. They watch the stories about Super PACs funding candidates and hear from retiring legislators who say that fund-raising now consumes much of their time in office, and sense that this issue is the one that the government really needs to deal with before anything else moves forward.

Middle class striving for security

Sanders also speaks to many middle-class Americans who don't feel that their future is secure and who are struggling every day to make sure that they don't fall on the wrong side of the growing economic divide.

For many years now, social scientists have demonstrated how middle-class Americans have become much less secure as a result of cuts to the social safety net and the exodus of good, secure jobs overseas while the separation between the rich and poor becomes more extreme.

Americans tune in when Sanders says that "It is the tragic reality that for the last 40 years the great middle class of our country -- once the envy of the world -- has been disappearing." His campaign, Sanders argues, is about "creating an economy that works for all, and not just the 1 %."

Sanders promises that as president he would double down on programs that benefit the middle class. He would fight for government policies that create incentives for job growth here in the United States and programs that help to elevate the economic health of working Americans, including progressive tax policies and a robust public works program to build the nation's infrastructure while giving people work. He has proposed raising the minimum wage to $15 an hour.

This is not radical at all. Republicans are also playing on these anxieties in the electorate, albeit with an agenda that has little room for government. Though Clinton has also attempted to tackle these concerns, her ties to controversial free trade policies and her being enmeshed in a campaign finance system that privileges wealthier Americans hurts her efforts. The record of President Bill Clinton in the 1990s, whose White House pushed his party toward market oriented policies and courted big business, looms large over her rhetoric.

While Bill Clinton did preside over an era of significant economic growth, the divide between the rich and poor accelerated in the 1990s. Americans in the lowest income brackets struggled. As Michelle Alexander argued in The Nation, African Americans confronted what was effectively a 42 percent unemployment rate and a social safety net that had been torn to shreds. The long-term cost of the deregulatory policies he supported, moreover would become clear when the risky business of Wall Street resulted in the devastating market crash of 2008.

A liberal, not a radical

Although he is a democratic socialist, much of his rhetoric is really just that of an unrepentant New Deal liberal. Sanders thinks government is a good, he supports the expanded use of government to help social conditions, and he believes that much of what federal officials do helps society. Many Democrats, inside and outside the base, are happy to finally hear a Democrat actually enthusiastically support the core Democratic idea. For too long, Democrats have struggled to avoid the "L" word and mimic Republicans.

For too long, the conventional wisdom has argued mistakenly that Americans reject government. We are children of Ronald Reagan, they say, seeing government as a problem not the solution. Many Democrats have agreed and have worked hard to push the party to the center. Bill Clinton famously said in 1996 that the era of big government was over. But the Sanders campaign is on to something. Polls have consistently shown that Americans like government much more than the pundits suspect.

When asked generally about government, Americans can be negative. But when asked about specific programs like Social Security or the minimum wage they jump with approval. If you listen to Sanders' speeches they often include a long list of things that government has and continues to do well. Though conservatives will argue this is radical, in many states, including red states, polls show something different.

The long tail of Iraq

Even on foreign policy, Sanders makes arguments that really resonate in the Democratic Party. Most important there is Iraq. Nothing looms larger in recent years that the decision to go into Iraq. For many Democrats, and Republicans as well, the war launched by the Bush administration was one of the most disastrous decisions of recent decades -- and we're still dealing with the consequences in the Middle East: 51% of Americans, according to Pew, still view the decision to go to war as a mistake. The numbers are down from 2014, but still a majority.

In 2008, Clinton learned that many Democrats were as angry with members of their own party who went along with this decision as they were with the administration. Barack Obama made this a theme. On this issue -- while in Congress -- Sanders decided at the most difficult moment to vote against the war. This decision will have considerable appeal when discussions turn to foreign policy, an area Hillary Clinton has believed to be one of her strengths. This is not just the left, but a large portion of the electorate who see this as a fundamental turning point, and mistake, after 9/11.

None of this is to say that Clinton can't mount a vigorous and effective comeback. Nor is it to say that many elements of Sanders' record, including his work with socialist organizations won't be a huge stumbling block in more conservative states.

But a strategy that simply relies on dismissing Sanders as left of center or quixotic won't work. His arguments don't match this image, and his ideas will continue to excite many Democratic voters.

To come back, Clinton will need to develop a more coherent and more compelling set of ideas that she can call her own.

Presidential campaigns are about inspiration. Julian Zelizer

Presidential campaigns are about inspiration. Americans understand the limits of what a president can do and the problems of our gridlocked political system, but they want to hear from candidates who are going to at least try to push for big changes from the start. A campaign based on the promise of fighting and "getting things done" will continue to have trouble against a campaign about a set of powerful ideas, a "political revolution," that makes sense to large portions of the American electorate.