(L to R) Former New York City mayor Rudy Giuliani stands with president-elect Donald Trump before their meeting at Trump International Golf Club, November 20, 2016 in Bedminster Township, New Jersey.

Three House committees on Monday began investigating reported efforts by President Donald Trump and his personal lawyer, Rudy Giuliani, to pressure "the government of Ukraine to assist" Trump's re-election campaign by having Ukraine probe former Vice President Joe Biden and his son, Hunter.

The investigation comes weeks after Giuliani reportedly urged a top representative of Ukraine's new president, Volodymyr Zelensky, to probe whether Ukrainian officials tried to harm Trump's 2016 campaign and whether Biden's own diplomatic efforts in that country affected Hunter Biden's involvement with a gas company in Ukraine owned by an oligarch there.

Giuliani said late month that State Department had helped him with his bid to pressure the Ukraine government to investigate the former vice president and the Democratic National Committee. Biden is leading fellow Democrats vying to run against Trump in 2020.

Biden campaign spokesman Andrew Bates in a statement to CNBC, said, "Putting himself before his country, Donald Trump solicited foreign interference in the 2016 election. His campaign's leadership also met with a foreign national offering 'high level and sensitive information' as 'part of Russia and its government's support for Mr. Trump.' Now he is abusing the office of the presidency and jeopardizing national security aid to a key ally in order to pressure them to prop-up a comprehensively discredited conspiracy theory that The Washington Post editorial board has labeled 'bogus on its face.'"

"This is beneath us as Americans, and it reinforces a truth that Trump and his allies have made patently clear for months: he is terrified of facing Joe Biden," Bates said.

The House investigation, which began with a letter demanding documents from the White House, is being led by the committees of Foreign Affairs, Oversight and Reform, and Select Committee on Intelligence.

"A growing public record indicates that, for nearly two years, the President and his personal attorney, Rudy Giuliani, appear to have acted outside legitimate law enforcement and diplomatic channels to coerce the Ukrainian government into pursuing two politically-motivated investigations under the guise of anti-corruption activity," the panels' chairmen wrote the White House.

"As the 2020 election draws closer, President Trump and his personal attorney appear to have increased pressure on the Ukrainian government and its justice system in service of President Trump's reelection campaign, and the White House and the State Department may be abetting this scheme."

The letter in a footnote cites a tweet in early August from Giuliani.

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The chairmen of those panels noted that the Trump administration has threatened to withhold more than $250 million in security assistance from Ukraine that has been appropriated by Congress at a time when "Ukraine's sovereignty and territorial integrity are under assault from Russia and its proxies" in Crimea and eastern Ukraine.

"If the President is trying to pressure Ukraine into choosing between defending itself from Russian aggression without U.S. assistance or leveraging its judicial system to serve the ends of the Trump campaign, this would represent a staggering abuse of power, a boon to Moscow, and a betrayal of the public trust," the letter to the White House says.

Giuliani dropped plans to visit Ukraine in May to urge then-president-elect Zelensky to follow through on probes into Hunter Biden and into disclosures before the 2016 U.S. presidential election of Trump campaign chairman Paul Manafort's consulting work for a pro-Russia political party in Ukraine.

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The White House, a Giuliani spokeswoman and Biden's campaign did not immediately respond to a request for comment by CNBC.

The new investigation comes as congressional Democrats and prosecutors already were conducting a number of probes into Trump, his associates, his finances and his business ties.

The latest inquiry is also part of a series of Democratic efforts to push for new election safeguards in the wake of the 2016 contest, when Russian operatives engaged in efforts to swing the election in Trump's favor.

On Friday, the House Judiciary and Oversight committees announced they would investigate possible violations of the so-called emoluments clauses of the Constitution, stemming from Vice President Mike Pence's stay at Trump's privately owned golf resort in Ireland on an official state visit and Trump's suggestion that another one of his resorts could be used to host next year's G-7 summit of world leaders.