Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.) slammed President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE on Friday over the lawyer's planned trip to Ukraine to promote investigations that he said will help Trump.

"It is highly unethical for the president’s personal lawyer to go meet with officials from a foreign government to see if they can influence, somehow, the upcoming presidential election," Warren said of the reported trip.

“Rudy Giuliani should just back off," she added.

Sen. Warren speaks in Kermit, WV: "It is highly unethical for the president's personal lawyer to go meet with officials from foreign government to see if they can influence somehow the upcoming presidential election. We've had enough of that & Rudy Giuliani should just back off." pic.twitter.com/z2ozUVB4pi — The Hill (@thehill) May 10, 2019

Giuliani told The New York Times on Thursday that he intends to ask Ukraine's president-elect to investigate the origin of the federal Russia probe, as well as former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE's past interactions with the country.

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The Hill reported details of Biden's relationship with Ukraine last month.

"We’re not meddling in an election, we’re meddling in an investigation, which we have a right to do,” the former New York City mayor told the Times.

“There’s nothing illegal about it,” he added. “Somebody could say it’s improper. And this isn’t foreign policy — I’m asking them to do an investigation that they’re doing already and that other people are telling them to stop. And I’m going to give them reasons why they shouldn’t stop it because that information will be very, very helpful to my client, and may turn out to be helpful to my government.”

According to the newspaper, Giuliani hopes to find information that will lessen the credibility of special counsel Robert Mueller's investigation, as well as the credibility of the case against former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE. He also hopes to find information that will hurt Biden's campaign.

Such investigations were previously launched by current Ukrainian President Petro Poroshenko, but he lost a recent election to political newcomer Volodymyr Zelensky, who is set to enter office June 3.

Biden reportedly threatened to withhold $1 billion from Ukraine in U.S. loan guarantees in 2016 unless the country got rid of a prosecutor, who was later voted out. His son, Hunter Biden, also served on the board of a company that the prosecutor had been investigating, the Times reported.

The trip follows the conclusion of the Mueller probe, which reviewed whether the Trump campaign coordinated with Russia to interfere in the 2016 presidential election. Mueller's report did not establish that Trump's campaign coordinated or conspired with Russia, but did detail extensive contacts between Trump associates and Russian figures.

Warren is among more than 20 Democrats competing for the party's 2020 presidential nomination and her campaign has seen a bit of momentum in recent weeks.