WASHINGTON (AP) — President Donald Trump urged the new leader of Ukraine this summer to investigate the son of former Vice President Joe Biden, a person familiar with the matter said Friday.

Democrats condemned what they saw as a clear effort to damage a political rival, now at the heart of an explosive whistleblower complaint against Trump.

It was the latest revelation in an escalating controversy that has created a showdown between congressional Democrats and the Trump administration, which has refused to turn over the formal complaint by a national security official or even describe its contents.

Trump defended himself Friday against the intelligence official’s complaint, angrily declaring it came from a “partisan whistleblower,” though he also said he didn’t know who had made it.

The complaint was based on a series of events, one of which was a July 25 call between Trump and Ukrainian President Volodymyr Zelenskiy, according to a two people familiar with the matter.

Trump urged Zelenskiy to probe the activities of potential Democratic rival Biden’s son Hunter, who worked for a Ukrainian gas company, according to one of the people. Trump did not raise the issue of U.S. aid to Ukraine, indicating there was not an explicit quid pro quo, according to the person.