Former Vice President Joe Biden announced Sunday he would adopt portions of two proposals from Bernie Sanders and Elizabeth Warren concerning free public college and bankruptcy, respectively, in an apparent bid to win over their supporters hours before Biden and Sanders were set to square off in the first one-on-one debate of the Democrats' primary season.

But, Sanders' campaign quickly rebuked the Biden initiative, saying it simply wasn't sufficient. The shift was somewhat unusual for Biden, as frontrunners typically have moved toward the political middle with an eye toward the general election, instead of embracing more left-wing ideas.

"Across the country, middle and working class families are being squeezed by debt," Biden tweeted Sunday. "This is a massive problem, and one that we need all of the best ideas to solve. That's why today, I'm adopting two plans from ⁦‪@BernieSanders⁩ and ⁦‪@ewarren⁩ to achieve this."

"First: Bernie has put forward a plan that would make public colleges and universities free for families whose income is below $125,000," he continued. "It's a good idea, and after consideration, I am proud to add it to my platform."

In response, representatives for Sanders reiterated that the Vermont senator didn't want any income test whatsoever for free college.

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"It's great that Joe Biden is now supporting a position that was in the Democratic platform four years ago," Sanders' team said in a statement. "Now, we have to go much further. We need to make all public universities, colleges and trade schools tuition-free for everyone like our high schools are. We need to cancel all student debt. And, we can fund it with a small tax on Wall Street speculation."

A tax on so-called "Wall Street speculation," Sanders' team previously said, "will raise an estimated $2.4 trillion over ten years" and, in one fell swoop, make all "public colleges, universities and trade schools tuition-free... and cancel all student debt over the next decade."

"It's great that Joe Biden is now supporting a position that was in the Democratic platform four years ago." — Bernie Sanders campaign

The proposal specifically would place a "0.5 percent tax on stock trades – 50 cents on every $100 of stock – a 0.1 percent fee on bond trades, and a 0.005 percent fee on derivative trades." Conservatives have hammered the idea as a de facto "tax on savings," noting it would tax many Americans' 401(k)s and retirement accounts.

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Biden said Sunday his proposal, part of Sanders' College for All Act of 2017, will "help roughly 8 out of every 10 families" and include a $70 billion investment in historically black schools. Biden went on to say his new plan would "require these schools to use the additional funds to lower costs for low-income and middle-class students," in addition to his previous plan to "make sure everyone can attend two-years of community college without debt" and to double the maximum value of Pell grants.

Biden's campaign did not elaborate on how the proposal would be funded.

"Second: Few people in the country understand how bankruptcy hurts working families more than Elizabeth Warren," Biden continued. "Today, I am fully endorsing and adopting her bankruptcy plan, which she called 'Fixing Our Bankruptcy System to Give People a Second Chance.'"

In an ill-fated virtual town hall on Friday that The Verge dubbed a "technical nightmare," Biden had announced his initial support for Warren's proposal, which aimed to simplify the bankruptcy process, saying it's "one of the things that I think Bernie and I will agree on."

Warren, in a Medium post earlier this year, said her plan would "make it easier for people being crushed by debt to obtain relief through bankruptcy," "expand people’s rights to take care of themselves and their children while they are in the bankruptcy process," end the "absurd rules that make it nearly impossible to discharge student loan debt in bankruptcy," and "let more people protect their homes and cars in bankruptcy so they can start from a firm foundation when they start to pick up the pieces and rebuild their financial lives."

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Additionally, Warren said her plan would "help address shameful racial and gender disparities that plague our bankruptcy system" and "close loopholes that allow the wealthy and corporate creditors to abuse the bankruptcy system at the expense of everyone else."

Sunday night's debate comes as the nation and much of the world struggle to contain a global pandemic amid mounting economic uncertainty.

Much has changed in the United States, and in the campaign, since the Democrats' last debate less than three weeks ago.

“Moments like these don’t come around often in campaigns, and this is a perfect opportunity to show millions that you have what it takes,” said Robert Gibbs, the former White House press secretary and campaign adviser to President Barack Obama. “They must show voters they are the answer to what is missing right now by being calm, honest, ready to lead and empathetic.”

Biden will step on stage as the front-runner, a distinction that seemed unlikely when Sanders was winning early contests. But, more moderate Democrats have consolidated rapidly around Biden, buoyed by his strong standing with black voters and motivated by a desire to block Sanders, as many of them signaled fears he'd lose to President Trump in November.

The fast-moving coronavirus largely has grounded the candidates for days, leading them to cancel campaign rallies even as big states prepare to vote on Tuesday. That's also frozen the trajectory of the primary contest, limiting Sanders' opportunities to regain momentum.

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Advisers said Biden also will aim in Sunday's debate to show voters who backed Sanders or other liberal candidates that they'd have a home in his campaign.

After rebounding in stunning fashion from his sluggish start in the primary, Biden has held a solid lead over Sanders in the all-important delegate race since. A strong showing in Tuesday’s primary contests essentially could guarantee his nomination. Four big states will be up for grabs: Illinois, Ohio, Arizona and Florida, a perennial general election battleground where Biden has appeared to have an edge over Sanders.

Fox News' Pat Ward, Andrew Craft, Madeleine Rivera and The Associated Press contributed to this report.