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HOUSTON — Joe Biden and Elizabeth Warren will finally square off on the debate stage tonight. The big question: Will they continue their long-running beef over bankruptcy law?

Biden, the moderate former vice president, and Warren, the populist Massachusetts senator, are at the top of the Democratic primary polls, along with Vermont Sen. Bernie Sanders. Tonight's debate, in Houston, will mark the first time they’ve stood side-by-side onstage. And if it goes as expected, it will be the most high-profile chapter in a long and sometimes fraught relationship.

Biden and Warren first clashed almost two decades ago, when Biden championed a bankruptcy overhaul bill that made it harder for some people to shed their debts. The bill was a top priority of the credit card companies headquartered in Biden’s home state, and after years of effort from Biden, it eventually became law in 2005. But not before he and Warren had it out repeatedly, including at a tense Senate Judiciary Committee hearing.

The bill would “take away the last shred of protection” from families facing bankruptcy, Warren warned Biden during that 2005 Senate hearing. He repeatedly grew frustrated with her during the exchange, eventually admitting with a wry grin, “You’re good, professor.”

Biden told Warren as he swore her into the Senate in 2013 that “you gave me hell” during the bill.

The two developed a friendlier relationship when Biden was in the Obama administration and Warren worked to help oversee the Wall Street bailout programs, create the Consumer Financial Protection Bureau and, later, serve in the Senate. In 2016, when Biden considered challenging Hillary Clinton, they had a private meeting where he floated the idea of Warren being his running mate.

Warren said Biden “was on the side of the credit card companies”

Since entering the presidential race, they’ve mostly avoided taking direct shots at one another. Both campaigns declined to talk on the record about their strategies for tonight’s debate. Biden and the bill's supporters have argued that it was necessary to help lower credit card interest rates and prevent people from gaming the system. His campaign pointed to changes that he helped insert into the final product.

"Joe Biden fought to secure critical concessions for working families in the bankruptcy bill, including protecting access to the ‘safe harbor’ of Chapter 7 forgiveness for those making less than the median in their state, and ensuring that child support and alimony were the top priority of debt payments -- ahead of the big banks,” Biden spokesman Michael Gwin told VICE News. “At the same time, Biden stood up to industry lobbyists to make sure that the law forced credit card companies to warn potential customers about their interest rates.”

The bill seems likely to come up at tonight’s debate, either by the moderators or the candidates themselves. It was at the center of the only major flare-up between the candidates so far. In late spring, Warren said Biden “was on the side of the credit card companies” in the fight, leading to a lengthy litigation of the bill between their campaigns.

Biden’s team has telegraphed that he might respond by highlighting her past corporate work. He’s also suggested that he’ll try to undermine her “plan for that” mantra, highlighting his record of legislative accomplishments and suggesting that campaign policy plans, no matter how detailed, don’t make change. He’s also almost certain to tangle with Warren and Sanders over Sanders’ Medicare for All proposal, which she supports, among other issues.

However all this goes down, it will mark the start of a new and more serious phase of the Democratic primary, with Warren, Biden and Sanders physically and metaphorically centerstage. Their economic policy differences may define the next few months of the race — and Thursday’s debate could set up that contrast for the remainder of the campaign.