He said he'll make one big point: He's held liberal positions on economic issues and civil rights for decades.

The first one he brought up: The Glass-Steagall Act, which separated commercial and investment banking until it was repealed under President Bill Clinton. He wants to re-institute it, but his rival Hillary Clinton has said she wouldn't. Progressives view re-instating that Depression-era law as a key part of reforming Wall Street.

Sanders said he'll highlight "which candidate has the track record and the history and the ideas of standing up to those very powerful special interests and fighting for an economy that works for all Americans rather than just the top 1%. And I think in that regard, Secretary Clinton and I have differences in terms of Wall Street."

The best evidence of Sanders' potency comes in polls out of Iowa -- where he is within single digits of Clinton -- and New Hampshire, where he's led since August. His seeming viability is causing some Clinton allies to wonder whether she'd be best paring back her presence and conceding the Granite State's first-in-the-nation primary so she could focus her efforts elsewhere.