Sanders is officially an independent, and he caucuses with the Democrats in the Senate. But he identifies as a Democratic socialist, and as the moniker would suggest, Sanders has planted himself decisively on the left wing of American politics during several decades in public office.
Perhaps more than any other issue, Sanders has elevated income inequality as the centerpiece of his presidential campaign. He supports raising the minimum wage, raising taxes on wealthy households and corporations, and a stronger social safety net. He's panned Democrats and Republicans alike for taking a soft approach to Wall Street, blaming the industry for the 2008 economic crisis and vowing to take a harder line on big banks if he's elected president.
Sanders is also a big supporter of organized labor, and he's criticized both parties for elevating the concerns of corporations over the needs of everyday workers. He takes a skeptical view of free trade agreements, because he worries they would outsource American jobs. And though he supports comprehensive immigration reform that includes a path to citizenship, he's warned that an influx of migrant labor could displace American workers.
On social issues like abortion and same-sex marriage, Sanders has been reliably progressive (though on another divisive social issue, gun control, his record has been more mixed.) And on foreign policy, he's tended to be dovish, generally backing diplomatic action over the use of military force.
It's a solidly populist record, and Sanders has used it to push his presidential candidacy into the big leagues, raising millions of dollars from small donations and drawing thousands of people to his rallies. While many are skeptical Sanders will ultimately be able to derail Clinton, the party's dominant frontrunner, there's no denying the energy surrounding the senator's bid.
Here's a look at where Bernie Sanders stands on the issues.
Sanders has criticized the soaring cost of higher education on the campaign trail, framing it as a perpetuator of inequality. He's offered a bill that would make tuition free at public colleges and universities, while allowing many who are currently paying back student loans to refinance.
"There would be no payment requirements for middle-class families, and no 10-hour workweek to add on to a student's class load," explains a blog post on Sanders' campaign website. "Students would be able to use federal, state and institutional need-based aid to cover room and board, books and living expenses - all major contributors to student debt."
He's also raised the possibility of reducing college costs by cutting the defense budget and redirecting the savings into the higher education system.
On elementary and secondary education, Sanders has generally backed a strong public education system. He's voted against bills that would expand the use of vouchers for private and charter schools. He's suggested teachers should be paid more than they currently are, and he's supported measures that would mandate a reduction in the number of students per class. He's pushed to expand early childhood education, calling it a priority that the U.S. has "undervalued" for far too long.
He voted against final passage of the No Child Left Behind law in 2001, criticizing the bill's focus on "high-stakes" standardized testing to measure student achievement and punish low-performing schools. He voted for an update to that law that passed a Senate committee in July that maintained the testing requirement but allowed individual states and districts to determine how much weight that testing should carry.
Sanders has said he supports comprehensive immigration reform that includes a path to citizenship for the roughly 11 million undocumented immigrants living in the U.S. But he has voiced concerns about one element of most comprehensive reform proposals - a guest worker program to permit foreign laborers to enter the country - warning that it would crowd native-born Americans out of the job market.
"I see the absolute need to provide legal status and protection to the undocumented people who are in this country now -- some 11 million people," he said last month duringan appearance at the Hispanic Chamber of Commerce. "But here's where I do have concerns: There is a reason why Wall Street and all of corporate America likes immigration reform, and it is not, in my view, that they're staying up nights worrying about undocumented workers in this country. What I think they are interested in is seeing a process by which we can bring low-wage labor of all levels into this country to depress wages for Americans, and I strongly disagree with that."
It was that concern about foreign laborers, in part, that pushed him to vote against a 2007 reform bill negotiated by Sens. John McCain and Ted Kennedy. But in 2013, he voted for another comprehensive reform bill that passed the Senate that included many of the same provisions.
Despite his concerns about guest workers, immigration reform advocates see Sanders as an ally on most other issues. He's labeled those who propose the forced deportation of undocumented immigrants "ugly beyond belief." He's voiced support for President Obama's executive actions to shelter some undocumented immigrants from deportation, saying he'd elect to go even further if the law permits. He also backed the DREAM Act, which would confer a path to citizenship on undocumented immigrants brought to the U.S. as children who agree to serve in the military or go to college.
Eyeing the widening gap between the wealthiest one percent of Americans and the rest of the country, Sanders has proposed a number of populist measures that aim to provide more support and economic mobility for middle- and lower-income workers.
He's endorsed a push to raise the federal minimum wage to $15 per hour, and he's proposed capping credit card interest rates at 15 percent. He proposed a bill to invest $1 trillion dollars in U.S. infrastructure, arguing it would create jobs and enhance America's competitiveness in the global economy. He also proposed a $5.5 billion measure to help employ Americans between 16 and 24 years of age "and provide job training to hundreds of thousands of others," according to his campaign website.
On taxes, Sanders has endorsed bills that would increase the tax burden on the wealthy by expanding the reach of estate taxes, levying a one-time "surtax" of 10 percent on billionaires, and ending tax loopholes that are enjoyed primarily by wealthy households, among other measures.
"We need a tax system which asks the billionaire class to pay its fair share of taxes and which reduces the obscene degree of wealth inequality in America," Sanders explained in a statement in June.
He's taken a tough line on Wall Street, blaming the deregulation of the big banks for enabling reckless activity that nearly tanked the American economy in 2008. He's authored a bill to break up "too big to fail" financial institutions, and he supports a measure from Sens. John McCain, R-Arizona, and Elizabeth Warren, D-Massachusetts, that would reinstate the Glass Steagall Act, a measure repealed in 1999 that separated investment banking from commercial banking.
Sanders opposes the Trans-PacificPartnership, a massive pending free trade agreement that's supported by the White House, some congressional Democrats, and nearly all congressional Republicans. Sanders has warned that the agreement would only hasten the outsourcing of American jobs and potentially undermine U.S. labor and environmental standards.
On health care, Sanders voted in favor of the Affordable Care Act, and he defended the law as "modest step forward" in a 2013 op-ed. But he would elect to go much further by establishing a "single-payer" health care system in which the government is the sole provider of health insurance, arguing the profit motive in the private insurance industry will only continue to undermine the integrity of the U.S. health care system.
On most social issues, Sanders has been a stalwart progressive. He strongly supports a woman's right to choose to have an abortion, for example, and he's criticized recent attempts by Republicans to defund women's healthcare provider Planned Parenthood.
"The current attempt to discredit Planned Parenthood is part of a long-term smear campaign by people who want to deny women in this country the right to control their own bodies," he said in a statement last month.
Sanders is an outspoken supporter of LGBT rights as well. He hailed the Supreme Court decision in June that legalized same-sex marriage nationwide, and in 2009, when Vermont became the first state in the country to legalize same-sex marriage legislatively, Sanders declared himself "proud" to represent the state in Congress. Unlike some other candidates, Sanders' support for gay equality isn't something he came by recently. He was speaking out in favor of equal treatment for gay people as early as the 1970s.
On gun laws, though, Sanders has cultivated a more moderate record, perhaps owing to his representation of a state with a rich tradition of hunting and firearm ownership. He voted against the 1993 Brady Bill, which mandated background checks for many gun purchases. He voted to shield gun manufacturers from lawsuits from victims of gun violence and their families in 2005. And in 2009 he voted to allow guns in checked baggage on Amtrak trains.
On other votes, though, Sanders stood in favor of stricter gun laws. He supported a ban on semiautomatic military-style assault weapons in 1994. He voted in 2013 to ban "high capacity" gun magazines that store over 10 bullets. And he voted in favor of a bill in 2013 that would have expanded background checks for gun purchases.
In his approach to global hot-spots, Sanders has generally supported diplomatic action over military action.
He did not vote to authorize the 2003 invasion of Iraq, and he's been critical of calls for greater U.S. involvement in the fight against the Islamic State of Iraq and Syria, saying other countries in the region need to to shoulder more of the security burden. He also opposed the limited U.S. intervention in Libya in 2011, criticizing the administration for taking military action without consulting Congress.
Sanders recently came out in support of the Iranian nuclear deal, which would scale back economic sanctions on Iran in exchange for limits on the country's nuclear energy program. "It's so easy to be critical of an agreement which is not perfect," Sanders told CBS News' "Face the Nation." "But the United States has to negotiate with, you know, other countries. We have to negotiate with Iran. And the alternative of not reaching an agreement, you know what it is? It's war. Do we really want another war, a war with Iran?"
Sanders has also spoken out in support of President Obama's push to normalize relations between the U.S. and Cuba, and he's said he would support ending the trade embargo between the two countries as well.