Sen. Bernie Sanders (I-Vt.) announced on Wednesday that he'll run for president of the United States in 2016.
"I am running for president," Sanders told The Associated Press. "People should not underestimate me... I've run outside of the two-party system, defeating Democrats and Republicans, taking on big-money candidates and, you know, I think the message that has resonated in Vermont is a message that can resonate all over this country."
Sanders will make a more formal announcement about his presidential campaign on Thursday.
Sanders is the first official challenger for the Democratic nomination to Secretary of State Hillary Clinton, who announced her candidacy earlier this month.
Sanders, a self-described "democratic socialist," has been hinting that he would seek the White House for nearly two years.
The senator, who has been in the upper chamber since 2007, previously served as Vermont's at-large congressman and as mayor of Burlington. He caucuses with the Democratic Party in the Senate and is categorized as a Democrat for the purpose of committee assignments.
In an interview with The Huffington Post last year, Sanders said that he wouldn't be dissuaded from entering the presidential race by the prospect of facing former Secretary of State Hillary Clinton in the Democratic primary.
"In terms of Hillary, I respect her. I've known her. I like her. So I'm not running to attack Hillary Clinton. I'm running to talk about the issues that impact the working class of this country and the middle class," he said last year.
Sanders, who has also characterized himself as "an independent Democrat," has expressed discomfort with the party, saying that it relies too heavily on "big-money interests" and doesn't adequately distinguish itself from the Republicans. But he has noted the "dilemma" of running for president outside the traditional two-party structure and the challenges of building an independent grassroots movement to do so.
Late last year, he visited Iowa, which will host the first party caucuses in 2016, to test enthusiasm for his candidacy.
Sanders has argued that the country deserves a real debate about policy issues, suggesting that such a conversation won't happen without a contested nomination process. "There's so much to be discussed, Ed, and we're not in this country about anointing anybody for a nomination," Sanders told MSNBC's Ed Schultz last year.
In the Senate, Sanders has focused on a range of issues, from reforming health care for veterans to addressing the threat posed by climate change. He has also been a strong advocate for a universal, single-payer health care plan nationwide, though the plan for such a system in his home state was recently dropped.