Top aides to Donald Trump sought on Sunday to turn the tables on Democrats pushing for his impeachment, insisting the president was the true "whistleblower" in pushing Ukraine to probe the son of rival Joe Biden for corruption.The Democratic-led House of Representatives launched an official impeachment inquiry this week after a whistleblower complaint led to the release of a phone call showing Trump pressed Kiev to probe his potential challenger for the White House.Trump's Republican allies have closed ranks around the embattled president, flatly denying he abused his power -- and his personal lawyer Rudy Giuliani, himself a key player in the scandal, led the charge with a series of combative talk show appearances on Sunday morning.Brandishing what he said were affidavits incriminating Biden's son Hunter over his role on the board of a Ukrainian gas company while his father was vice president, Giuliani said Trump had been duty bound to push Kiev to investigate."If he hadn't asked them to investigate Biden, he would have violated the constitution," Giuliani said on ABC's This Week."The president is the whistleblower here," echoed Trump's advisor Stephen Miller on Fox News Sunday. "Getting to the bottom of a corruption scandal in Ukraine is in the American national interest," Miller said. "The president of the United States is the whistleblower and this individual is a saboteur trying to undermine a democratically elected government."As Barack Obama's vice president, Joe Biden and other Western leaders pressured Ukraine to get rid of the country's top prosecutor, Viktor Shokin, because he was seen as not tough enough on corruption.Trump has claimed Biden was seeking to protect his son -- tweeting a Trump-approved ad attacking the Bidens' conduct - but that allegation has largely been debunked.Facing the biggest crisis of his presidency, Trump insists he did nothing wrong when talking to Ukrainian President Volodymyr Zelensky on July 25, characterising the impeachment push as a new "witch hunt."In tweets to his 65 million followers over the weekend, he lashed out at Democrats as "savages" and called for lawmaker Adam Schiff, who is leading the impeachment probe, to resign from Congress.Between Saturday night and Sunday morning, the president retweeted dozens of video clips from Republican allies defending his conduct and criticising Democrats - while warning in a message to supporters that "our country is at stake like never before."