DES MOINES, Iowa >> Allegations that President Donald Trump courted foreign interference from Ukraine to hurt his leading Democratic rival, Joe Biden, dominated presidential politics Saturday, as Biden demanded a House investigation of Trump’s phone call in July with Ukraine’s leader and as Trump lashed out, denying wrongdoing without releasing a transcript of the call.

With Trump seizing on a familiar defense, saying Democrats were undertaking a “witch hunt” against him on Ukraine, Biden called on the House of Representatives to begin a new investigation of whether the president sought the interference of a foreign government to bolster his reelection campaign.

“This appears to be an overwhelming abuse of power,” Biden said during a campaign swing in Iowa. “We have never seen anything like this from any president.”

Trump is said to have urged the Ukranian president on a July 25 phone call to investigate Biden’s son, Hunter Biden, who did business in Ukraine while his father was vice president. Trump’s request is part of a secret whistleblower complaint in the intelligence community that is said to involve Trump making an unspecified commitment to a foreign leader, according to two people familiar with the complaint.

The sharp accusations between Trump and Biden elevated the president’s dealings with Ukraine as a potentially significant new issue in the presidential race, and offered voters a preview of what is likely to be an extraordinary general election contest if Biden were to win the nomination.

The controversy has focused on whether Trump manipulated foreign policy — a military aid package to Ukraine had been delayed at the time of the phone call — to pressure the country’s newly elected president, Volodymyr Zelenskiy, to take action to damage Biden’s election bid.

On Saturday, Trump sought to deflect attention from that question by accusing Biden of acting improperly as vice president in calling for the ouster of a Ukranian prosecutor who had overseen an inquiry into corruption related to the oligarch whose company employed Hunter Biden.

Trump described his conversation with Zelenskiy as “perfectly fine and routine.”

“Now that the Democrats and the Fake News Media have gone ‘bust’ on every other of their Witch Hunt schemes, they are trying to start one just as ridiculous as the others, call it the Ukraine Witch Hunt,” Trump wrote on Twitter. He said that any effort to investigate him would fail, comparing it to the investigation by Robert Mueller, the special counsel, into his ties to Russia during the 2016 campaign.

Intensifying a line of attack he and his allies have stoked for months, Trump said the real problem was Biden and questions about what the president described as “the Joe Biden demand that the Ukrainian Government fire a prosecutor who was investigating his son.”

Referring to his conversation with Zelenskiy, Trump said: “Nothing was said that was in any way wrong, but Biden’s demand, on the other hand, was a complete and total disaster.”

No evidence has surfaced to support Trump’s claim that the former vice president intentionally tried to help his son by pressing for the prosecutor general’s dismissal. But some State Department officials had expressed concern that Hunter Biden’s work in Ukraine could complicate his father’s diplomacy there.

The issue strikes a particular nerve for Biden, who has long feared putting his family under the harsh spotlight of a presidential campaign. During a two-minute encounter with reporters on Saturday morning, he grew irate, angrily insisting that he had never spoken with his son about any overseas work.

“You should be looking at Trump,” Biden said. “Trump is doing this because he knows I’ll beat him like a drum.”

Even as they avoided mentioning Biden, other Democratic presidential candidates moved quickly to capitalize on the new dynamic in the race. Sen. Elizabeth Warren, who rarely mentions Trump in her stump speech, opened her remarks at a cattle call on Saturday afternoon by excoriating both the president and congressional Democrats.

“He has solicited another foreign government to attack our election system,” she told a crowd of 1,200 cheering Democratic voters gathered in Des Moines for an afternoon of primary speeches. “It is time to call out this illegal behavior and start impeachment proceedings right now.”

Warren, who first called for Trump to be impeached in April after the release of Mueller’s report, went further on Saturday, arguing on Twitter that by failing to act on impeachment, Congress had become “complicit in Mr. Trump’s latest attempt to solicit foreign interference to aid him in U.S. elections.”

Although he has yet to call for impeachment proceedings against Trump — as have several of his rivals for the 2020 Democratic nomination — Biden on Saturday tiptoed closer to embracing the idea that has been gaining support on Capitol Hill despite opposition from House Speaker Nancy Pelosi.

Biden, whose appearances on the campaign trail can be halting and sprinkled with misstatements, has generally delivered his strongest performances when focused on Trump. Speaking about the president allows Biden to discuss foreign policy and national security, issues that his campaign has said differentiate Biden, a former Senate Foreign Relations Committee chairman, from the rest of the 2020 Democratic field.

While Trump’s attacks give Biden the one-on-one showdown with the president that his campaign has spent months trying to create, it also exposes him and his son to another round of questions about Hunter Biden’s business activities in Ukraine.

The Biden campaign moved quickly to warn the news media over the story, underscoring a deep concern about how allegations about the younger Biden’s work will be received by voters. “Any article, segment analysis and commentary that does not demonstrably state at the outset that there is no factual basis for Trump’s claim, and in fact that they are wholly discredited, is misleading readers and viewers,” said the deputy campaign manager, Kate Bedingfield, in an email to reporters.

But Biden advisers also seized on the furor to portray Trump as fixated on, and worried about, a potential general election race against Biden.

“There is only one candidate the president is trying to get foreign governments to dig up bogus dirt on,” Anita Dunn, a senior adviser to Biden, said.

The effort by the president and his team to shift the focus to Biden could boomerang, casting the Democratic front-runner as a sympathetic figure unfairly attacked with foreign help.. It could just as easily mark a defining moment for Biden, a 76-year-old politician first elected to the Senate in 1972 and long accustomed to playing by the more genteel political rules of a different era.

Donna Brazile, the former Democratic National Committee chairwoman who led the party through Hillary Clinton’s loss to Trump three years ago, said the exchange on Saturday “in many ways feels like 2016.”

Just the prominent discussion of the actions of Biden and his son in Ukraine, regardless of the merits of the president’s accusations, has the potential to hurt Biden, Brazile said.

“We’re basically creating a political story which right now is undermining Joe Biden when I do believe the real focus should be getting the substance of the complaint out to the American people as soon as possible,” she said.

On Saturday morning, Trump posted a video mash-up of TV news footage of stories about Biden’s son. “This is the real and only story,” the president wrote.

Citing the reports by journalists seemed contradictory given the president’s claim that the media had not reported on the issue. But the tweets signaled that Trump and his campaign organization would be doing as much as possible to sow doubt about Biden.

Sen. Chris Coons of Delaware, a Biden backer, said that no matter who emerges as the Democratic nominee in 2020, that person will face misinformation and slashing personal attacks by the president and his campaign.

“You have any other nominee, Trump will do something comparable to try to disadvantage that nominee,” said Coons, as he walked with Biden into a Polk County Democratic Party event. “I don’t see anything about this story that is specific to Joe Biden.”

So far, Biden’s rivals, nearly all of whom converged on Iowa this weekend, have been quick to assail Trump while avoiding commentary about how the president’s accusations against the Bidens would affect the Democratic nominating contest.

Sen. Cory Booker of New Jersey, whose campaign manager on Friday released a memo stating he would have to drop out of the race if he failed to raise $1.7 million before the end of September, remarked that “this is not a partisan issue.” He and former Rep. Beto O’Rourke of Texas reiterated their calls for Trump to be impeached.

Even if Biden’s primary competitors don’t take direct aim, the perception of Biden’s son leveraging his connections cuts a stark contrast with two leading rivals, Warren and Sen. Bernie Sanders, who have centered their candidacies around a fierce populist message of rooting out corruption in Washington.

It’s a message that worked in 2016 for Trump, who cast Clinton as the avatar of establishment self-dealing, a past-her-prime creature of Washington unable to adjust to the times and to produce real change.

Biden’s team is acutely aware of that comparison. After Biden initially gave only a meager retort to the issue Friday, his campaign decided to go further.

Sensing an opportunity to highlight Trump’s fixation with Biden, his campaign released a statement in his name condemning the president for “abhorrent” conduct and demanding Trump release the transcript of his call with the Ukrainian leader and allow the director of national intelligence to release the whistleblower’s claims to Congress.

Advisers to Biden said his initial reluctance to comment reflected his prudence about discussing sensitive national security matters rather than unease with the work of his son in Ukraine. But the former vice president is highly sensitive about questions regarding his family, and it was not until several outlets reported on what Trump said in the phone call that the Biden campaign determined it should try to go on the offensive.

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