VPR News has learned from several sources that Independent U.S. Sen. Bernie Sanders will announce his candidacy for the Democratic presidential nomination on Thursday.

Sanders will release a short statement on that day and then hold a major campaign kickoff in Vermont in several weeks.

Sanders' entry into the Democratic race ensures that Hillary Clinton will face a challenge to win the support of the liberal wing of the party.

Sanders' basic message will be that the middle class in America has been decimated in the past two decades while wealthy people and corporations have flourished.

His opposition to a proposed Trans-Pacific Partnership trade deal (T.P.P.) shows how he plans to frame this key issue of his campaign.

"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 said in a phone interview on April 23.

As the longest-serving Independent member of Congress, Sanders has been a vocal critic of the influence that large corporations have on the political process.

"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.

And Sanders says it is imperative that all the Democratic presidential candidates address the issue of trade and income inequality.

"So, I think that Hillary Clinton and every candidate out there should in fact address whether or not they support this T.P.P.," Sanders said.

In the past few months, Sanders has been actively visiting many of the early presidential primary states. Just last weekend he traveled to South Carolina to address the state Democratic Party and news reports indicate that his economic message drew a lot of support at the state meeting.