(Adds Wall Street Journal report, comment from Biden and Democratic chairmen, link to explainer)
By Jonathan Landay, Aram Roston and David Morgan
WASHINGTON, Sept 20 (Reuters) - A political battle over a classified whistleblower complaint about U.S. President Donald Trump escalated on Friday, with Democrats warning of a national security threat and Republicans turning it into an attack on Joe Biden, one of Trump's chief political rivals.
Trump dismissed the complaint from a whistleblower within the intelligence community - reported by several U.S. news organizations to involve the Republican president's communications with a foreign leader - as a partisan hit against him.
The Sept. 12 complaint centered on Ukraine, the Washington Post reported.
Reuters has not confirmed details of the whistleblower's complaint. But a source familiar with the matter said it alleged "multiple acts" by Trump, not just a phone call with a foreign leader. The source requested anonymity because of the sensitivity of the issue.
Trump had spoken with Ukraine's recently elected president, Volodymyr Zelenskiy, less than three weeks before the complaint was filed. Trump is due to meet Zelenskiy during a United Nations gathering in New York.
The July 25 call between the leaders is under investigation by three Democratic-led House committees, who want to know if Trump and his personal lawyer, Rudolph Giuliani, tried to pressure the Ukrainian government into aiding Trump's re-election campaign.
The Wall Street Journal reported on Friday that Trump urged Zelenskiy about eight times during the call to work with Giuliani to investigate Biden's son.
Giuliani said on CNN on Thursday he had asked the Ukrainian government to investigate Hunter Biden, the son of former Vice President Biden, who is a frontrunner in the field of Democrats seeking to challenge Trump in the 2020 presidential election.
Giuliani alleged that as vice president, Biden sought the firing of a Ukrainian prosecutor who was investigating his son's business dealings. Biden and his son have denied the charge.
"Not one single credible outlet has given any credibility to his (Trump's) assertion - not one single one. And so I have no comment except for the president to start to be president," Biden told reporters in Cedar Rapids, Iowa.
The former prosecutor, Viktor Shokin, had been criticized by the U.S. government and the European Union for larger issues, including blocking reforms to Ukraine's legal system. Ukraine's parliament approved his dismissal in March 2016.
On Friday, Giuliani was seen by Reuters reporters at the Trump International Hotel, a few blocks away from the White House, sitting next to Lev Parnas, a Ukrainian businessman with whom he has recently been working. Giuliani declined comment.
In a Sept. 9 letter to White House Counsel Pat Cipollone requesting documents, the chairmen of the House foreign affairs, intelligence and oversight committees said the Ukrainian government's readout of Trump's call appeared to show that he encouraged Zelenskiy to pursue the Biden investigation.
The chairmen noted the State Department had acknowledged that Kurt Volker, the U.S. special representative to Ukraine, subsequently arranged for Giuliani to meet an aide to Zelenskiy in Spain.
The letter also cited news media reports that Trump threatened to withhold more than $250 million in security assistance approved by Congress for Kiev to aid its fight against Russian-backed separatists in eastern Ukraine.
Trump did not mention a provision of foreign aid to Ukraine on the July 25 phone call with Zelenskiy, the Wall Street Journal reported, citing an unnamed source.
Trump said he did not know the identity of the whistleblower or the precise accusations but that all of his conversations with foreign leaders had been appropriate.
"It doesn't matter what I discussed but I will say this: somebody ought to look into Joe Biden's statement because it was disgraceful, where he talked about millions of dollars that he is not giving to a certain country unless a certain prosecutor is taken off the case. Somebody ought to look into that," Trump said.
The dispute is the latest chapter of a power struggle in which the Trump administration has been resisting efforts by Democratic lawmakers investigating the president's business dealings and actions to obtain documents, records and testimony from White House and senior agency officials.
An intelligence community watchdog determined that the whistleblower complaint was credible, related to an urgent matter, and should be shared with congressional leaders through a process laid out by U.S. law.
That determination was overridden by acting Director of National Intelligence Joseph Maguire after consulting with the Justice Department.
Democratic House Speaker Nancy Pelosi said the administration was violating federal law by "stonewalling" a congressional inquiry.
"Reports of a reliable whistleblower complaint regarding the president's communications with a foreign leader raise grave, urgent concerns for our national security," Pelosi said. "If the president has done what has been alleged, then he is stepping into a dangerous minefield with serious repercussions for his administration and our democracy."
The Democratic chairmen of the House intelligence, oversight, judiciary and foreign affairs committees criticized Trump for attacking the whistleblower as partisan, saying it was a "brazen effort to intimidate" the person and could chill future complaints.
A senior Democratic member of the House Judiciary Committee, which is considering whether to impeach Trump, said the incident could feed into the panel's deliberations.
"This is deadly serious," Representative David Cicilline said on Twitter. "If the President does not allow the whistleblower complaint against him to be turned over to Congress, we will add it to the Articles of Impeachment."
House Republican leader Kevin McCarthy echoed Giuliani's allegations about Biden and said the whistleblower could have brought his or her complaint directly to Congress, telling reporters, "I want to know who the whistleblower is, what they are saying. But they could have come to Congress and given it to us."
In response, Eric Swalwell, a Democratic member of the House Intelligence Committee, tweeted "false." Swalwell said a whistleblower would be charged with a crime if they shared classified information with lawmakers without authorization.
(Reporting by Steve Holland, Jan Wolfe, David Morgan, Mark Hosenball, Jonathan Landay, Aram Roston, Eric Beech, Mohammad Zargham in Washington; Trevor Hunnicutt in New York; Amanda Becker in Cedar Rapids, Iowa; Writing by Roberta Rampton; Editing by Howard Goller and Tom Brown)
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