Former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE slammed President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE on Twitter on Tuesday after Trump showed off a folded piece of paper to reporters that he said contained an unannounced component of his recent deal with Mexico.

"President Trump is bringing his secret one-page agreement with Mexico to Iowa," Biden tweeted as he and Trump campaigned in the first-in-the-nation caucus state.

President Trump is bringing his secret one page agreement with Mexico to Iowa. The Iowans being crushed by his tariffs would like to see it. — Joe Biden (@JoeBiden) June 11, 2019

Trump showed off the paper to reporters as he departed the White House for the Hawkeye State.

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"That’s the agreement that everybody says I don’t have," the president said, pulling the document from the pocket of his jacket.

The U.S. and Mexico came to an agreement last week for Mexico to take additional steps to lessen the flow of migrants northward after Trump threatened to slap a 5 percent tariff on all Mexican goods. Trump says the deal includes a component that has yet to be revealed, though Mexican officials have denied that is the case.

The war of words between Trump and Biden, the front-runner for the 2020 Democratic presidential nomination, is intensifying.

The former vice president is slated to call Trump "an existential threat to America" in a speech in Davenport, Iowa, on Tuesday evening, according to prepared remarks.

Trump hit back against the remarks Tuesday, calling Biden "failing" and "slower than he used to be."

"When a man has to mention my name 76 times in a speech, that means he’s in trouble," the president told reporters.