While prognosticators have suggested that the slate of primary states voting later in March could be fertile ground for Biden's coalition of voters, Sanders suggested that the former vice president could struggle to run on his record in states that he said "have been hit very hard" by trade woes.

“Joe is going to have to explain to the people, the union workers in the Midwest why he supported disastrous trade agreements" like NAFTA that Sanders said have "cost this country millions of good paying jobs, and in fact have resulted in a race to the bottom.”

Sanders went on to needle the former vice president on his past votes for the Iraq War and the Wall Street bailout. And he dinged Biden's opposition to Medicare for All, calling for a debate "talking about why the United States is the only major country on Earth not to guarantee health care to all people through something like" Sanders' signature health care proposal.

He also hammered home attacks on Biden's decades-old advocacy for freezing spending on social safety net programs, echoing attack ads he launched earlier in the day.

“At a time when half of our people are living paycheck to paycheck and struggling to make ends meet,” Sanders told reporters, “Joe is going to have to explain to the American people why he voted for a disastrous bankruptcy bill, which benefited the credit card companies.”

After cruising as the party's clear frontrunner through the first four nominating contests, Sanders on Wednesday found himself for the first time with a true rival for the Democratic nomination. Unlike Sanders, who won three of the first four nominating contests, Biden struggled through the primary's early races. But the former vice president won a commanding victory in South Carolina, where he was powered overwhelmingly by the support of black voters.

That momentum carried the former vice president into Super Tuesday, when he won primaries all over the country, including dominating wins in the South and upset victories in Texas and Massachusetts. His performance, along with Sanders's wins in California, Colorado and Utah, put the two men on a collision course for the Democratic nomination, leaving the only two remaining candidates, Sen. Elizabeth Warren and Hawaii Rep. Tulsi Gabbard, in the dust.

Sanders on Wednesday railed against the consolidation of Democratic establishment support behind Biden after his more moderate rivals, with whom he'd split much of the vote, dropped out and endorsed him this week. He also homed in on Biden's support from wealthier Americans, asserting that "at last count, [Biden] has received funding from at least 60 billionaires."

"Joe is running a campaign, which is obviously heavily supported by the corporate establishment," Sanders argued, touting his own campaign's status as a grassroots fundraising behemoth.

"So what does it mean when you have a campaign which is funded very significantly by the wealthy and the powerful?" he asked. "Does anyone seriously believe that a president backed by the corporate world is going to bring about the changes in this country that working families and the middle class and lower income people desperately need?"