(CNN) Joe Biden stormed into the Democratic primary with a laser focus on the man he wants to sweep out of the White House next year.

But before Biden can book a prize fight with President Donald Trump, he's going to have to contend with a party -- and 20 other candidates -- with very different ideas about the way forward, and, in the case of Sen. Elizabeth Warren, an opponent from bygone years ready to renew their old clashes.

The Biden-Warren rivalry traces back some two decades, when the then-future vice president was entrenched in the US Senate and Warren, then a law professor at Harvard, took frequent and often pointed aim at Biden and other lawmakers she viewed as too cozy with corporate interests. Now, as the pair vie for the Democratic presidential nomination, it is Warren again who seems among the most eager to question Biden's past -- and his future plans.

Despite being born only about six-and-half years apart, Biden, 76, and Warren, 69, have come to symbolize distinct and competing factions within the Democratic party. Along with her ideological ally, Vermont Sen. Bernie Sanders , Warren has been at the forefront of a progressive resurgence determined to unmoor the old establishment, pushing it left with the promise of deliverance from a "corrupt" and increasingly unequal economic order. Biden, meanwhile, is pitching himself as an experienced bipartisan statesman and safe harbor for Democrats -- and maybe some Republicans -- worried, as he is, that another term of Trump threatens to "forever and fundamentally alter the character of this nation."

Like most of the Democratic candidates in the 2020 field, Warren has so far largely steered clear of commenting on -- let alone, attacking -- her rivals.

But when asked last week whether the former vice president was "too cozy" with Wall Street, the Massachusetts senator was unusually direct, noting that their "disagreement is a matter of public record."

"At a time when the biggest financial institutions in this country were trying to put the squeeze on millions of hardworking families who are in bankruptcy because of medical problems, job losses, divorce and death in the family, there was nobody to stand up for them," Warren said in Cedar Rapids, Iowa, on the same day that Biden officially announced his candidacy.

And then, with a single sentence, Warren opened up a new chapter in the campaign: "I got in that fight because they just didn't have anyone, and Joe Biden was on the side of the credit card companies."

A feud, renewed

Through the late 1990s and into the 2000s, Warren -- best known then as an academic and author -- fought against a Biden-backed bill to overhaul the country's bankruptcy laws.

The two brought starkly different philosophies to the debate, with Warren launching broadsides in the op-ed pages while Biden helped move the legislation along in the Senate.

At a moment when the country's bankruptcy rates were on the rise, Biden backed legislation that would make it more difficult for Americans to discharge their debts. Warren vehemently opposed that proposal, saying it would only give more power to lenders like credit card companies that were already taking advantage of financially distressed individuals and families.

As she fought against that bill, year after year, Warren directly called out Biden on numerous occasions. In op-eds, blog posts and other writings, Warren accused Biden -- nicknamed back then by critics as "the senator from MBNA," the name of a credit card issuer -- of catering to the industry, which had its power base in his home state of Delaware.

She also made a sharp argument based on gender. In an essay published in the Harvard Women's Law Journal in 2002, she warned that the bill Biden supported would most hurt women, describing them as the "largest demographic group in bankruptcy."

"Senator Biden can publicly support one very visible piece of legislation on behalf of women, stratifying his duty and assuring the loyal support of million of women," Warren wrote, referring to the Violence Against Women Act that Biden helped write. "He is then free to be a zealous advocate on behalf of one of his biggest contributors, the financial services industry, and still position himself as a champion of women."

Warren and Biden's long battle would come to a head in 2005, as the bill, revived after Bill Clinton left office, was again wending its way through the Congress.

Senator vs. Professor

At a Senate Judiciary Hearing that February, Warren sat in the witness seat and delivered an impassioned case against it. In a sharp exchange, Warren argued that the credit card industry was already piling on debtors with "interest and fees and payments," provoking a reaction from a frustrated Biden.

"Maybe we should talk about usury rates, then," he said. "Maybe that's what we should be talking about. Not bankruptcy." When Warren suggested she would be happy to, Biden added, "Let's call a spade a spade. Your problem with credit card companies is usury rates from your position. It's not about the bankruptcy bill."

But Warren insisted on a connection, arguing, "if you're not going to fix that problem, you can't (with the bankruptcy bill) take away the last shred of protection from these families."

At that, a smiling Biden quipped: "I got it, OK. You're very good professor."

The tension seemed broken, but Biden's support for the bill remained intact. Congress passed it and it was signed into law by President George W. Bush that April.

In a statement to CNN, Biden campaign spokesman Andrew Bates said that the former senator had done his best to contend with a harsh political reality. By 2005, he said, the bill's passage was -- given the GOP's control of Congress and the White House -- a "certainty."

"Then-Sen. Biden fought for and won important concessions for middle class families in it, including protecting access to Chapter 7 forgiveness for working people, making child support and alimony the number one priority for debt payments -- in front of big banks and credit card companies -- and forcing credit card companies to warn borrowers about their interest rates," Bates said, adding that Biden "is proud of his record as a lifelong champion of economic dignity."

More than a decade after their Capitol Hill clash, in 2018, as Biden and Warren were considering their presidential runs, the senator from Massachusetts introduced a bankruptcy bill of her own -- one that would, quite literally, hit home for Biden.

The legislation would, in effect, force companies to file for bankruptcy in courts where their business is functionally based, rather than using a loophole to bring the cases where the company is incorporated -- typically in more friendly jurisdictions. Warren said in a press release that her proposal would "prevent big companies from cherry-picking courts that they think will rule in their favor and to crack down on this corporate abuse of our nation's bankruptcy laws."

The bill, which would likely become a priority in a future Warren administration, could have an outsized effect on New York and Biden's home state of Delaware -- two of the most popular states for companies looking for big businesses seeking bankruptcy protections.