Former Vice President Joe Biden indicated on Tuesday that he would spend his time at the fourth Democratic debate focusing on someone who won't be on the stage: President Trump.

"President Trump is leading the most corrupt administration in modern American history. It’s got to end," Biden tweeted hours before taking the stage in Westerville, Ohio.

"We must restore integrity to our government and ensure our elected officials work for all Americans — not just the powerful," he added.

From the outset of his campaign, Biden -- one of the Democratic frontrunners -- has focused his energies on criticizing Trump. Whatever enmity there is between the two may have intensified after President Trump took aim at both the former vice president and his son's business dealings.

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During a rally on Friday, Trump called Hunter Biden a "loser" and crudely claimed the elder Biden only did well because he sucked up to former President Obama.

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On Monday, the former vice president released an ethics plan that was a direct response to Trump's presidency.

"Donald Trump is the definition of corruption," he tweeted. "At every turn, he has abused the power of the presidency to benefit his bottom line and weaponized the Executive Branch to serve himself. I’ll ensure our government works for the people -- not just Donald Trump."

Biden's pitch to Democratic voters is centered around his claim that he is the only Democrat guaranteed to defeat Trump in November 2020, an apparent nod to voters who worry that infighting will hurt any chance to finally remove the president from office.

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Biden has also come under fire from the more progressive wing of the party. His health care plan, for example, has received criticism for not reforming the system enough, although he seeks to build on Obama's Affordable Care Act.

Sen. Elizabeth Warren, D-Mass., often seen as a more progressive alternative, could use Tuesday's debate to attack Biden and solidify the recent lead she's gained over him in some polls.