Vermont independent Senator Bernie Sanders is expected to announce Thursday that he will challenge Hillary Clinton for the Democratic nomination for U.S. president.

"Bernie for a while has said he's going to make a decision at the end of the month," a close Sanders adviser said, reported the Guardian. "On Thursday, he'll put out a statement. Unless something dramatically changes in next 48 hours, he's decided he's going to run for president, and he'll be launching his campaign in May up in Vermont."

The adviser told the Guardian that Sanders plans to mount "a real campaign that will raise significant resources" by focusing fundraising efforts "on low dollar donors online."

Sanders recently called the current level of income inequality "grotesque" and vowed to limit campaign donations from wealthy donors, saying he supports public financing of campaigns "so you don't have to spend your time hustling money to run and can actually talk to your constituents," reported Bloomberg.

Sanders, 73, is a self-described Democratic socialist who was first elected to the House of Representatives in 1990, going on to win a seat as Senator in 2006. He's the longest-serving independent in congressional history, and caucuses with the Democrats in the Senate, where he serves as the ranking Democrat on the budget committee.

His presidential announcement would make him the first person to challenge Clinton for the Democratic nomination, and he's expected to pressure the former secretary of state from the left on issues like free-trade pacts, Wall Street regulation, income inequality and Social Security, according to USA Today.

"If you want to understand why the middle class in America is disappearing and why we have more wealth and income inequality in America than we have had since the late 1920s, you have to address the issue of trade," Sanders told Vermont Public Radio.

"All of the major corporations want to continue with this trade policy. Wall Street wants to continue this trade policy. The drug companies want to continue this trade policy. But organizations representing American workers and the environment do not want to continue the trade policy. They want new trade policies," he said. "So, I think that Hillary Clinton and every candidate out there should in fact address whether or not they support this T.P.P [Trans-Pacific Partnership]."

Speaking on the issue at an anti-TPP event last week, Sanders said, "She's going to have to be clear ... are you on the side of working people who would suffer as a result of this disastrous trade agreement, and seeing their jobs go to China or Mexico, or are you on the side of corporate America? It's not a very difficult choice," according to CNN.

Still, Sanders insists that he's "not running against Hillary Clinton," but rather, "I'm running for a declining middle class."

Sander epitomizes the progressive plight, advocating to address climate change, same-sex marriage, higher taxes on the wealthy and civil liberties. He also supports a single-payer, universal health care system and large infrastructure reinvestment programs. And while Clinton voted to authorize the Iraq war in 2002, Sanders, then a member of the House, voted against authorization.