Bernie Sanders, the liberal senator from Vermont, has announced he is officially running for president, which creates a challenge for the Democratic frontrunner Hillary Clinton.

Sanders, an independent in the Senate, has been contemplating a run the past several months, traveling to early states to gauge support for his progressive vision of eliminating income inequality, combating climate change and creating universal healthcare.

“For many months I have been traveling from coast to coast across our country, and have had the opportunity to meet with thousands of good, hard-working, and remarkable people. Like you and me, they are deeply concerned about the future of our country,” Sanders said in an email to supporters Thursday morning announcing his campaign.

Sanders made his intention to run official first in interviews Wednesday evening with USA Today and the Associated Press. He told those organizations that people “should not underestimate” him and he is running “this election win.”

“After a year of travel, discussion and dialogue, I have decided to be a candidate for the Democratic nomination for president,” Sanders said in the email.

“But let’s be clear. This campaign is not about Bernie Sanders. It about a grassroots movement of Americans standing up and saying: ‘Enough is enough. This country and our government belong to all of us, not just a handful of billionaires,’” he added.

Sanders, 73, grew up poor in Brooklyn, New York before attending Brooklyn College and later The University of Chicago. He was mayor of Burlington, Vermont in the 1980’s and then represented the state’s at-large district in the U.S. House of Representatives from 1991 until 2007.

In 2006, Sanders was easily elected to the U.S. Senate from Vermont, defeating his Republican opponent with 65 percent of the vote. He was reelected in 2012, pulling in over 70 percent of the vote in the contest.

During his tenure in the Senate, Sanders has authored legislation to combat climate change, been an outspoken supporter of universal healthcare for U.S. citizens and has been critical of economic policy he says only benefits top earners.

Sanders famously spoke for over eight hours on the Senate floor in opposition of extension of the Bush-era tax cuts in late 2010. Most recently, he was praised for co-authoring legislation with Arizona Sen. John McCain to reform Veteran’s Healthcare at VA hospitals while he was head of the VA committee.

Sanders has also been critical of attempts to cut or change social welfare programs, such as Social Security. Just Wednesday, the Des Moines Register ran an op-ed from Sanders where he called for an expansion of the program.

Sanders has been traveling to early voting states to meet with potential supporters of a campaign. He has made several swings through Iowa, including two stops in Ames where he delivered keynote addresses in front of Story County Democrats.

In a December event in Ames, Sanders told a crowd of over 200 that his supporters would need to launch a “political revolution,” if he runs and candidates need to be prepared to take on the “millionaire and billionaire class.”

While traveling, Sanders has been relatively quiet when it comes to criticisms of Clinton. But recently he has ratcheted up attacks, saying it is “unlikely” Clinton will combat income inequality, an issue Sanders will likely campaign on. He has also criticized the Obama’s administration’s support for “fast track” approval of the Trans-Pacific Partnership trade deal.

Current plans have Sanders holding a larger campaign rollout in his home state of Vermont in the coming weeks. He is reportedly set to travel to New Hampshire, an early primary state, as early as this weekend.