Independent Sen. Bernie Sanders of Vermont said Wednesday he will run for president as a Democrat, promising to fight what he deems "obscene levels" of income disparity and vowing to do more than just raise progressive issues.

“I believe [voters] want a fundamental change so that government works for ordinary Americans and not just billionaires,” Sanders told USA Today. He said he would make the announcement official on Thursday.

The self-described “democratic socialist” from the small Northeastern state of Vermont enters the race as a robust liberal alternative to Democratic front-runner Hillary Rodham Clinton, the former secretary of state and first lady.

Sanders pledged to do more than simply nudge Clinton to the left in a campaign in which she is heavily favored.

“People should not underestimate me,” he said in an interview with The Associated Press. “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 told USA Today he can compete by attracting small contributions from millions of Americans and mobilizing young people and other volunteers to help him wage a nationwide campaign.

“I am running in this election to win,” he said. “Most people in America have never heard of Bernie Sanders. More than 90 percent of Americans have heard of Hillary Clinton ... I will absolutely be outspent. But I do believe we have a chance to raise significant amounts of money through small individual contributions.”

He added that although he has always run in Vermont as an independent, it made more sense to work within the Democratic primary system “where it’s much easier to get on the ballot and have a chance to debate other candidates.”

Sanders is the first challenger to enter the race against Clinton, who earlier this month became the first Democrat to formally declare her intention to run for president. He is likely to be joined in the coming months by former Maryland Gov. Martin O'Malley, former Virginia Sen. Jim Webb and former Rhode Island Gov. Lincoln Chafee.

Sanders caucuses with Democrats in the Senate. With liberal Democrats such as O'Malley and Sen. Elizabeth Warren of Massachusetts, he has been pressing Clinton to move to the left on economic policy by embracing tighter Wall Street regulation and a more robust social safety net.

Sanders has turned up the heat on Clinton over the Trans-Pacific Partnership (TPP) and a proposal to fast-track the approval of the 12-nation trade pact. He has blasted the TPP as a jobs killer that favors corporations.

As he has for months in prospective campaign stops and throughout his political career, Sanders on Wednesday assailed an economic system that he said has devolved over the past 40 years and eradicated the nation's middle class.

Sanders, 73, rejected the idea his appeal is limited to voters on the left, predicting Wednesday that his message would appeal to both fellow independents and Republicans.

Sanders said he would release “very specific proposals” to raise taxes on wealthy Americans and corporations, as well as offer tuition-free education at all public colleges and universities. He touched on his past opposition to free-trade agreements, his support for heavier regulations of the U.S. banking industry and his vote against the Keystone XL oil pipeline as a preview of his campaign.

“What we have seen is that while the average person is working longer hours for lower wages, we have seen a huge increase in income and wealth inequality, which is now reaching obscene levels,” he told the Associated Press.

“This is a rigged economy, which works for the rich and the powerful and is not working for ordinary Americans ... You know, this country just does not belong to a handful of billionaires.”

Wire services