Wisconsin Democratic U.S. Sen. Tammy Baldwin called on Republicans to call former national security adviser John Bolton to testify in the impeachment trial of President Donald Trump on Monday, saying it was "gut check time" for senators who had sworn oaths to defend the United States Constitution.

Baldwin also singled out her Republican counterpart, Wisconsin U.S. Sen. Ron Johnson, saying what Bolton reportedly wrote in his unpublished memoir about Trump's position on U.S. aide to Ukraine contradicted what the president told Johnson last August.

The New York Times first reported on the Bolton manuscript over the weekend, stating a draft of the book had been sent to the White House on Dec. 30 to be reviewed for classified information. The manuscript reportedly states Trump told Bolton in August he wanted to continue freezing $391 million in security assistance to Ukraine until officials there helped with investigations into former Vice President Joe Biden and his son Hunter Biden.

Speaking Monday during a Washington, D.C. press conference with U.S. Sen. Minority Leader Chuck Schumer, Baldwin said senators deserved to hear Bolton describe what he wrote under oath.

"If Ambassador Bolton provided the White House with evidence about President Trump withholding congressionally authorized and mandated U.S. security assistance to help Ukraine fight Russian aggression in exchange for political investigations from Ukraine to benefit President Trump personally, then that would constitute direct evidence of abuse of power," Baldwin said.

Baldwin also referenced a letter Johnson sent to House Republicans in November as House Democrats were leading an impeachment inquiry into the president.

In the letter, Johnson stated Trump had called him Aug. 31. Johnson said in the letter that Trump told him he did not withhold financial assistance to Ukraine in order to force leaders there to investigate the Bidens.

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Baldwin said Bolton's manuscript directly contradicted what the president told Johnson.

"It is my hope that my Republican friends would want to know if there is evidence that President Trump did not tell one of my colleagues the truth," Baldwin said. "The people of Wisconsin certainly want to know if the president did not tell Sen. Johnson the truth."

A spokesperson for Johnson did not immediately respond to a request for comment.

Trump took to Twitter Sunday night to deny the reports.

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump tweeted. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

While Monday's remarks from Baldwin were her most forceful yet on impeachment, Johnson has been an outspoken opponent of the effort for months, arguing his firsthand knowledge of the Ukraine funding proved Trump had legitimate reasons for withholding U.S. aide to Ukraine.

Johnson's position as chair of the Senate Homeland Security and Governmental Affairs Committee since 2015 put him in a position to repeatedly visit Ukraine. It also put him in contact with members of Trump's inner circle who are key to the Democrats' impeachment arguments.