Born in Brooklyn, with an accent to match, Mr. Sanders ran unsuccessfully in the 1970s for governor and United States senator in Vermont before being elected mayor of Burlington in 1981. For 16 years, he served as the only congressman in the state before he was elected to the Senate in 2006.
Mr. Sanders has been a modest legislator and something of a lone wolf in Washington, promoting largely the same legislative agenda since his early days as a mayor. He voted against the Iraq War and, in 2008, he was one of roughly two dozen senators to vote against the $700 billion bailout of big banks.
And while he is often viewed as a pesky left-wing gadfly, he is also known to reach across the aisle, working on legislation with Senator Charles E. Grassley of Iowa and Senator John McCain of Arizona, both Republicans. He has rationalized voting for the 1994 crime bill, now heavily criticized for some its draconian provisions, by saying he had favored progressive parts of the bill, including the Violence Against Women Act, while strongly opposing measures that would lead to mass incarceration.
Mr. Sanders is the longest-serving independent in congressional history, a point of pride for him but one of consternation and annoyance for some Democrats who are quick to suggest he does not have the party’s interests at heart. Some Democrats blame him for Mrs. Clinton’s loss in 2016, saying his anti-establishment rhetoric during his campaign inflamed divisions in the party that proved insurmountable.
Mr. Sanders largely avoided scrutiny during his 2016 presidential run but he will likely face more direct attacks from his opponents and more attention from the news media in a second bid for the White House.
One 2016 campaign issue that will almost certainly resurface is his record on gun control, Democratic strategists have said, given the intensity of the debate around gun violence following recent mass shootings. In 2005, Mr. Sanders voted for a law that granted immunity to gun manufacturers and dealers from most liability lawsuits. Mr. Sanders has also come under fire for support he received from the N.R.A. when he was running for Congress in 1990, in part because he vowed not to support a bill that mandated a waiting period for handgun sales.
Though his message is well worn, Mr. Sanders has indicated that he is trying to remedy weaknesses from his first presidential campaign. In recent months, he has made a series of trips to the South, where in 2016 he drew less than 20 percent of the black vote. On the Martin Luther King Jr. holiday this year, he made a two-day swing through South Carolina — where black voters made up about 60 percent of the Democratic primary vote in 2016 — that included addressing supporters and students and speaking with lawmakers.