Rudy Giuliani Rudy GiulianiGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Juan Williams: Breaking down the debates Giuliani criticizes NYC leadership: 'They're killing this city' MORE on Monday ripped former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE as an opportunist after a report on Bolton's forthcoming book said it contained damaging allegations against President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE and sharp criticism of Giuliani.

Giuliani, who serves as Trump's personal attorney, leapt to the president's defense after The New York Times reported that Bolton wrote in his manuscript of an August meeting in which Trump said he did not want to release security aid for Ukraine until the country agreed to investigate his political rivals.

"There is no way in the world President Trump would say this to John Bolton," Giuliani tweeted. "It’s a shame that a man will sacrifice his integrity to make a few bucks on a book. No wonder he accomplished so little as National Security Advisor."

.@LouDobbs There is no way in the world President Trump would say this to John Bolton. It’s a shame that a man will sacrifice his integrity to make a few bucks on a book. No wonder he accomplished so little as National Security Advisor. — Rudy Giuliani (@RudyGiuliani) January 27, 2020

Trump has denied that he told Bolton that aid for Ukraine was tied to investigations, and the president's allies have raised suspicions about the timing of the report given Bolton's book is due to be released in two months.

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But the report has roiled the impeachment proceedings in the Senate, where Trump and his defense team were hoping for a quick acquittal. The Bolton allegation has led to renewed calls from Democrats to hear Bolton testify.

Giuliani has been a central character in the impeachment saga as well. Multiple current and former administration officials testified last year that the former New York City mayor was conducting a shadow foreign policy in Ukraine to dig up dirt on Democrats, and that he led an effort to oust former Ambassador to Ukraine Marie Yovanovitch Marie YovanovitchGrand jury adds additional counts against Giuliani associates Lev Parnas and and Igor Fruman Strzok: Trump behaving like an authoritarian Powell backs Biden at convention as Democrats rip Trump on security MORE.

Bolton reportedly wrote in the manuscript that he raised concerns about Giuliani to other administration officials and suggested that Giuliani may have wanted Yovanovitch removed from her post because she was targeting his private clients as part of an anti-corruption agenda.

Bolton left the White House last September amid tensions with Trump and other staffers.