Allegations that President Trump courted foreign interference from Ukraine to hurt his leading Democratic rival, Joseph R. Biden Jr., dominated presidential politics on Saturday, as Mr. Biden demanded a House investigation of Mr. Trump's phone call with Ukraine's leader and as Mr. Trump lashed out, denying wrongdoing without releasing a transcript of the call.
With Mr. Trump seizing on a familiar defense, saying Democrats were undertaking a "witch hunt" against him, Mr. 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 re-election campaign.
"This appears to be an overwhelming abuse of power," Mr. Biden said during a campaign swing in Iowa. "We have never seen anything like this from any president."
Mr. Trump is said to have urged the Ukranian president on a July 25 phone call to investigate Mr. Biden's son, Hunter Biden, who did business in Ukraine while his father was vice president. Mr. Trump's request is part of a secret whistle-blower complaint in the intelligence community that is said to involve Mr. Trump making an unspecified commitment to a foreign leader, according to two people familiar with the complaint.