Democratic presidential frontrunner Joe Biden told reporters Monday that he does not think it's necessary for him to testify as a witness in President Trump's impeachment trial because he has "nothing to defend."

"This is all a game," Biden said at a campaign event in Iowa just a week away from the first primary. "Even, if they bring me up, no one has said that I've done anything that was wrong. Period. What is there to defend?"

His remarks come a day after an unpublished manuscript of Trump's former national security advisor John Bolton's book was leaked, where Bolton reportedly claims that Trump explicitly linked a hold on military aid to Ukraine to an investigation of Joe and Hunter Biden.

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The bombshell accusation against Trump -- which the president denied happened -- has reignited the furor from Democratic senators calling for witnesses, including Bolton, to testify in the ongoing impeachment trial.

Biden said Monday that Bolton's testimony is integral to the Senate's trial.

Democrats have long held that Bolton's firsthand knowledge of the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky in which Trump allegedly tried to pressure Zelensky to investigate whether Biden urged the firing of a prosecutor looking into a Ukrainian energy company his son Hunter was on the board of as possible corruption.

The phone call is the crux of an ongoing impeachment trial, which examines allegations of abuse of power and obstruction of Congress by Trump.

"The reason he is being impeached is because he tried to get a government to smear me," Biden said.

Reports that either of the Bidens would be potentially offered as witnesses in exchange for key testimony from a White House figure such as Bolton or Trump's chief of staff Mick Mulvaney has been shut down by Senate Minority Leader Chuck Schumer, D-N.Y., as well as House Intelligence Committee Chairman Adam Schiff, D-Calif.

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Biden has said he refuses to turn the trial into a "farce or some kind of political theater," according to reports by the Washington Post.